Hello, well with colours, maybe try to think of the mood you want it to express overall to work with the picture. A lot of colour illustrations I like to use a vibrant pink of some kind as the base layer then pick and work with colours on top of that that would make it suit the overall feel of what you want to express. Then there is when you want it to be more moody or dark so choosing more subdued colours (but can still make them vibrant or pop, like in this picture for example: https://ranfren.neocities.org/log/log6/kitsuner.png).

Don’t always choose pink and try to work more with blues at times but really like using pink can’t help it lol

A lot of the process is also just playing around and tinkering throughout, so its part of the fun (´・ω・｀) Also if there is an art piece or artist you like a lot, you can study it as well and try to replicate it as a study/practice.

Hope this helps in some way~