Basic colour theory covers three categories: the colour wheel, colour harmony and colour context.
Here's a colour wheel:
Colour harmony is crucial as it prevents an image from being boring, but also from being chaotic and unpleasant to look at.
I have been working on my own practice speedpaintings, but unfortunately I can't access them from my college at the current time... So I'm going to use some examples I found on this website.
Complementary colours sit opposite each other. Opposing colours create maximum contrast and make their counterpart "pop" in an image.
Colour context is noting how a colour "acts" when it is placed alongside other colours.
For example, here, the red appears more vivid against the blue-green than the orange, and similarly it appears brighter against black than white.
Understanding colour relativity in great detail allows you to fully manipulate and understand how to make certain shades stand out.
Now all I need to do is put this into practice!