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The skin colour of my subject is widely inconsistent and I am not sure what the reason for it is. To me it looks like the side of the face suffers from a colour shadow thrown by the bright hair. At the same time, the skin colour closer towards the centre of the face seems like an exaggeration of minor differences in colour of the subject's skin. Those reddish/pink spots aren't really visible to my eye.

What is the best approach to avoid such issues as I am taking a photo so I don't have to try and fix it in Photoshop afterwards? Is it something with my settings?

Picture is taken in JPG in manual mode with automatic white balance and spot-metering. No filters or special camera modes.

You are likely not getting the colors you want because of the color profile on your camera or raw processing software does not match your preferences. Camera and lens selection may also play a role. Lighting as well (see Michael C's answer).

Lens Selection

Lenses may transmit different frequencies differently. For instance, some produce warmer colors, while others are cooler. Some lenses also have defects, such as "glow", when shot wide open. Some people find that a soft-focus look is pleasant in portraits.

Auto white balance in some cameras normalizes color differences between lenses. In other cameras, it does not. Setting custom white balance typically neutralizes many lens color differences.

Camera Settings

If you are disciplined, you should set custom white balance. However, if you shoot in conditions with varied lighting, forgetting to change white balance can result in dozens of subsequent shots being ruined or wasting your time in post processing. (How significant this is depends on your workflow.)

Some cameras allow color-shift adjustments to auto white balance. Since AWB on my camera tends to produce images with more magenta than I'd like, I adjust AWB to increase the complementary color, green.

You should also select the color profile on your camera that best matches your preferences. Usually one or two profiles will subdue magenta-red colors. Available options vary by camera maker:

• 
  FujiFilm: Provia, Velvia, Astia, Classic Chrome, ProNeg-High, ProNeg-Low.


• 
  Canon: Standard, Portrait, Landscape, Neutral, Faithful.


• 
  Nikon: Standard, Neutral, Vivid, Portrait, Landscape, Flat.


• 
  Olympus (Picture Modes): i-Enhance, Vivid, Natural, Muted, Portrait.



• Sony (Creative Style): Standard, Vivid, Neutral, Clear, Deep, Light, Portrait, Landscape, Sunset, Night Scene, Autumn leaves.




• _{(Feel free to suggest edits with camera makers and profile names...)}

Cameras also typically allow adjustments to saturation, contrast, highlight, shadows, noise reduction, and sharpness.

Post Processing

The objectionable skin tones are most likely magenta, less so red. Even if the colors are accurate, you may still prefer less magenta.

Many tools do the same thing. The concepts underlying curves and levels apply to nearly every other color adjustment tool. Aside from that, use whatever tool is available or that you like best. Regardless of what you choose, it's helpful to know which primary and complementary colors go together (Red-Cyan, Green-Magenta, Blue-Yellow), as well as distinguish Red-Magenta and Blue-Cyan.

• You can reduce the magenta by adjusting curves to increase the complementary color, green, in relevant areas. Use color-layer blending to avoid changing the overall luminosity of the image. Use layer masks to isolate changes.




• You can do the same thing by adjusting levels. You can use the "auto" button or select white, gray, and black points with the dropper tool. Then look at each channel to see what the software did to use as a starting point for your own adjustments.




• Sometimes desaturating slightly is enough to fix skin tones. Use a layer mask to isolate changes.




• Michael C states that he likes HSL/HSV/HSB adjustments. I don't use it at all because I don't know how to adjust "Hue" to get intended results consistently.




• You can also try adjusting temperature and tint. (These, along with saturation, are the main color adjustment tools available in Google Photos.)




• Some editors include skin-tone specific tools. If these are available to you, experiment with them until you find settings you like.

In your sample image, the "whites" of the girl's eyes are pinkish. I used the gray dropper of the levels tool to select a point within her sclera. This gave RGB gamma adjustments of (0.71, 1.08, 1.26). Different points give slightly different values, but this is just a starting point, so it doesn't have to be "perfect".

I changed the gamma values to (0.85, 1.08, 1.1) because I thought the result was too blue/cyan. Then I changed it to (0.85, 1.15, 1.1) to decrease the magenta a bit. Her cheeks are still pinkish, but it should appear more natural and perhaps more to your liking.

Another approach is to use auto white balance on a copy of the image. Then use color layer blending to preserve luminosity. Adjust opacity of the color layer to taste. (I used 50%.) The result is slightly different. (This is an extended version of Laurence Payne's approach.)

What kind of light was illuminating your subject?

It all starts with the light.

Always.

I've found that most of the time uneven skin tones are a result of:

• Too many different types of lighting mixed from different angles, so that one part of skin is influenced more by, say, the tungsten lamp to the right while another part of the skin is more influenced by the fluorescent light high overhead to the left. Color casts from highly reflective objects, such as your subject's pink wig or even a rich green lawn on a sunny day, can shift the color of human skin.


• Poor overall lighting, even if all of it is the same type, with a low CRI (color rendering index). If there is no light present that is the same hue as parts of your subject's skin, there will be no light that color reflected from your subject. The colors that are present in the light will then be exaggerated.


• Flickering lighting, such as most "energy efficient" light sources like cheap fluorescent or LED lighting, combined with fairly fast exposure times so that as the opening between the shutter curtains transits the sensor, the intensity and color of the light is changing as the light cycles from the peak to the trough of the alternating current powering it. Flickering lights not only get brighter and dimmer with the cycle of the current powering them, they also go from bluer and fuller spectrum at peak to browner and lower spectrum at the trough.

What is the best approach to avoid such issues as I am taking a photo so I don't have to try and fix it in Photoshop afterwards? Is it something with my settings?

Control your light sources. Be sure you are using even, full spectrum lighting that does not flicker. Be sure that all of the light coming from different angles is the same, both in terms of color temperature along the amber ←→ blue axis and "tint" along the green ←→ magenta axis.

Control your camera's color. Be sure the camera's white balance setting matches your light. Again, this includes both color temperature adjustment as well as white balance correction along the "tint" axis.

Shoot raw anyway. As counterintuitive as this sounds, saving the raw data will allow you to make a few, simple global adjustments in post processing to dial your color in even more precisely than the coarser settings on your camera will allow. Your raw converter's HSL/HSV/HSB tool¹ can be invaluable here! You won't be forced to do various localized adjustments to a JPEG image near as often by processing the raw data.

For further reading here at Photography SE:
CFL and LED mixed
Why and how capturing RAW image instead of JPEG helps with editing
Why can software correct white balance more accurately for RAW files than it can with JPEGs?

This answer to How to edit photos shot in fluorescent light has some detailed examples of how an HSL tool was used globally (applied to the entire image without having to use individual adjustments to different areas) to adjust for the color casts to a wide variety of skin tones shot under less-than-full-spectrum lighting.

_{¹ HSL stands for 'Hue-Saturation-Luminance'. Other image processing applications call the same type of tool 'HSV (Hue-Saturation-Value)' or 'HSB (Hue-Saturation-Brightness)'. Luminance/Value/Brightness are all pretty much the same thing.}

Photoshop may seem like a bit of a drudge, but makeup aside, there's not a lot you can do about complexion problems when shooting colour. And complexion problems are the main issue here. Fortunately, the fix is a quick and easy one, taking under two minutes if you have a reasonably suitable computer and a little bit of practice under your belt.

The key is targeted (and masked) Hue/Saturation adjustment layers. Use the hand tool from the adjustment layer's setting panel to click on the worst area of redness, then select the "minus" tool (because the first step will have selected all of the skin to one degree or another) and click on a patch of "good" skin. Slide the hue adjustment crazily to one side to highlight the selected area with weird colour. Use the colour sliders at the bottom to rein in the selection to get as much "good" skin showing as you can manage. Then reset the hue adjustment to centre, and pull it towards green gradually until things even out as much as desired. Then mask out lips, eyes, and whatever else (in this case, hair) that would have been caught up in the colour selection (it's easier than painting in the skin you want to fix in a tight portrait). It helps if you have a tablet, but you can do it with a mouse if that's what you've got to hand, if a little more slowly. There are several YouTube tutorials using this technique; just search for "targeted adjustment photoshop". Once you know the technique and learn to spot where it will be useful and where the best clicks can be made in the image for targeting the colour problem(s), it can save you an enormous amount of post-processing time.

This is a quickie - I haven't put in a lot of care here (and would have left more of the hair reflection in some places to make it look more natural if I had taken more care - you do need the reflections to make it look right, and not the like the hair colour was Photoshopped in):

It's certainly a very warm colour balance. Is this more accurate? It's a simple 'Auto Color' process, available in most photo editors.

But, although it's refreshing to see a young girl not plastered in makeup, it IS rather cruelly accurate.

What shall we do about the shadow under the cheekbone? Maybe do some subtle work with the airbrush. Or just re-shoot, but reflect some light into that area?