There are so many factors that should work together to make a good painting. The artist cannot just concentrate on composition and nothing else. Or he cannot just concentrate on descriptive brush work and nothing else. He has to also consider form, drawing, values and the underlying abstract design. And then there is colour! As an artist you have to be able to mix the infinite number of colours that the creation displays with only a few tube colours. Of cause you cannot really do that, but by the right use of colours (tube and mixed) you can create the impression of millions of colours. The study of colour is a whole science in itself. I am only beginning to understand colours and relation between different colours.
If you are an artist and you want to learn more about colour, I can really recommend the work and material of a famous New Zealand artist, Richard Robinson. By visiting his website and registering for the FREE lessons would be a very wise step. You can also buy his DVDs and printable books on painting and especially colour in oil painting.