Colour didn’t come easily to me. Back when I was starting out – and to be honest, for quite a while after – often after I had finished a painting, I would look down at my palette, with its random blobs of pigment messed half into each other, and think “Why can’t my painting look like that?” It seemed that in the process of trying to get colour under control, it was very easy to kill it.
But after a long while I began to get a sense of how to approach colour; a few rules of thumb for working with it. Like all rules in painting of course they’re only guidelines, but they help.
The first thing to know about colour is that it’s possible and necessary to understand its three axes: value, hue and chroma (at least, to use Munsell’s terminology). I can’t claim credit for this discovery, of course, it goes back two or three hundred years. But it’s the absolute foundation for understanding colour composition.
Value can be understood as the darkness or lightness of a colour: is it a light blue or a dark blue? Chroma can be understood as the intensity of a colour: is it an intense blue or a washed out blue? And hue is what we usually mean when we say colour: is it blue at all? Or something else like red or green?
One good rule for composing with colour is that you can usually vary any two of value, hue and chroma in a single work and still wind up with a unified painting. So vary hue and value, but not chroma, for example: as long as your colours are all of approximately the same intensity, you can use all sorts of different hues and values. Or varying hue and chroma but keeping value consistent is another option, resulting typically in a very decorative effect. And less typically, some painters will vary value and chroma but not hue so much.
Where beginning painters run off the rails of course is that they typically vary all three in the same painting, which inevitably makes their paintings look like they were painted by beginners. Recall that painting is about ordering the visual elements at your disposal: maintaining a constant value, hue or chroma is one way of establishing a relationship between the other two axes.
Another thing to understand is that neutral tones, like white and black and grey, typically fall outside the “vary any two” restriction. So you could have a painting where you vary hue and chroma in the colours, but value varies (obviously, since you can’t have a dark white or a light black) in the neutrals, and that would be OK.
Finally, a word about colour symbolism: non-objective painters always hated this, but I don’t see how one gets away from it: colours have associations. If you paint a blue background for your painting, it will likely at some level read as “sky”. And if you want to paint a brick wall and have it read as such, you should use some variant of terra cotta rather than, say, green. And speaking of green, it has the inevitable association of growing, plantlike things, so if you want to evoke a gritty urban milieu you’d do best to stay away from it. The trick is to make this work for you: start your colour composing with the colours that make sense in the context of what you’re trying to represent or express. It doesn’t mean you have to adhere to a rigid symbolism, but just that it’s better to work with the associations, rather than trying to deny them or force them some way. Unless of course that’s the whole point.