The colour balance adjustment layer is a relatively simple tool for adding tones of colour to your image, rather than altering the colours themselves.
It works similarly to a White Balance adjustment, however instead of adjusting the white balance of the whole image, it works by adjusting the C-M-Y spectrum based on a selected light tone, for examples, affecting just shadows, midtones, or highlights.
This allows us to, for example, just make the highlights of an image warmer.
Or cool down the shadows.
Of course, you could just make adjustments to the midtones and use this tool as creatively as you want, to pretty quickly and easily get pink, purple and blue tones across your photo (it is generally a quicker tool to achieve these “fantasy” colours compared to the selective colour tool).
Or, you can use it to fix colour-casts on small areas, eg., if you notice there is a green colour cast, add magenta (the opposite of green) to cancel it out.
You can use it if you think you didn’t quite get the WB correct at the early stages and if you don’t want to (or can’t) make changes to your RAW file via the Camera Raw Filter.
One thing I’ve found is that when adding yellow, the image will take on a green tone. I tend to add 1/2 as much magenta for whatever value of yellow I add.
Eg., if I add -20 yellow, I will also add approx -10 Magenta.
As always you can mask this effect in or out, as you decide what you want to affect.
Adding some yellow and magenta to midtones or shadows can be a good way to make very dark/black eyes have more brightness and colour! Just remember to try and keep it natural, don’t go overboard!