DISCLAIMER: These are my own personal preferences, not set rules!
If you want to highlight or mute out certain colours in your picture, overlay/soft light helps with that! [YES I LOVE OVERLAY AND SOFTLIGHT]
For example, if you have a picture where you really want to accent the red, set a layer over your image -> set to overlay or soft light -> fill it in with brown, tan, muted orange.
* Filling it in with shades of red tends to be overkill, because it morphs the other colours in your piece to reflect red and it tends to look icky. Personal preference though.
* To MUTE red, you can fill your overlay/soft light layer mode w/ shades of blue.
* Blue in general is very good at muting/desaturating an image. It tones down warm colours, such as red, very well.
* Blue is also good for bringing out green hues. This is because blue is very close to green on the colour wheel so it complements it very well.
*Yellow is also good for bringing out green hues. Yellow is close to green on the colour wheel.
*Brown is very good at warming up an image in a vintage-y sort of way. I find that very light brown is good at tying colours together without actually warping the image’s original pallet. If you want blue to be the focal point of your image I don’t recommend adding a brown overlay/soft light layer because it will warp the blue into a warmer purple. Why purple? Because purple is a warmer variation of blue due to its closer proximity to red.
*Adding a grey overlay/soft light layer helps saturate/up the contrast of your original colour pallet.
*If you use purple/pink/green/yellow I recommend using a very muted shade and/or lowering the opacity, because these pigments can severely warp your original colour pallet. [but I still recommend trying it out just to see what works for you]
*Light/muted pink is a nice colour to brighten up an image.
