Okay a lot of what I know is just my having been fucking around. But a useful trick I find is this:
- Take a photo of a thing.
- Turn the saturation and, if you like, contrast WAAAAYYYYYy up.
- AHA now you can SEE the SNEAKY HIDDEN COLOURS in SHADOWS
The more grey/dingy a thing is the more colours are involved so you can just like, layer colours until it looks right. Two types of grey/dinge: the Object is grey, and the Lighting is grey. If the Object is grey, it'll probably pick up on the colour of light a bit more, depending on the texture of the thing/the grime on the thing. If the Lighting is dingy, make Every Object that dinge, add the teeniest bit of colour where light meets shadow, and then put highlights with the dinge colour slightly more saturated (but not too much more towards white). Sometimes saturation takes the place of lightness, when doing dinge.
If lighting is red, shade in blue is a good rule of thumb. This gets really fun when you have two light sources of different colours; all three primary colours? You get RAINBOW SHADOWS (think that 3d red blue effect, not actual stripey rainbow shadows (unless you wanna!)). Two secondary colours which just mix to brown? You can add more colour by taking a primary, probably the one they both share, and subtly putting it where that grey blur join happens.
Try making a piece using only fully-saturated colours, but at a low opacity/only colouring lightly, and layering. The photo saturation trick helps with this.
Try making a piece with a base colour other than black/pencil, and using other colours to highlight. Fiddle with how the colours will be way too bright if you don't bring them a bit towards the base colour; grey looks green when on red, but yellow/orange when on blue!
Good luck, have fun, colour is one of my favourite bits!!! I love to talk about it so if any of this is helpful or needs clarification or anything I'd love to!!!