If you have stained your canvas with a tint derived from your color scheme, you will see that unlike painting directly on to a white surface, the colors you start with don't seem so out of place. They sit nicely on the canvas. On a white surface they would stand out sharply and only settle back after you had covered the canvas completely. This is another advantage of staining your canvas with an underpainting. It allows you to judge your color better.
Whatever you do, it is a good idea to paint in the largest shapes first and then the lesser shapes, finally finishing off with all the smaller stuff, detail and so on. You can try to paint up all the detail from the word go, but it is not recommended. The difficulties in controlling and shaping your picture are intensified if you do.
If you paint up one part too thoroughly and find that when you have finished coloring your canvas it doesn't fit with the other parts, you have to go through all the business of scraping it out and repainting it. The best approach is to paint up each part, but only up to a point, leaving the final surface details until the very last when you are sure that everything is working together.
With your color schemes and cartoon drawings always by your side, you need have no fear in letting yourself go so that in turn the picture will let itself go as well. There comes a point in any painting when, however carefully you have worked out what you want to do, the painting wants to take over itself. Let it. From this point onwards all the surprises and excitements start happening. Should the picture run away with itself and get out of hand, you can always go back to the original idea contained in the cartoon.
You will see from all this that you will have to be more patient in the initial stages than with other forms of painting and drawing. If you take care with all the stages of building up your picture you will not only gain valuable knowledge about painting, and get into good workmanlike habits, but you will be more assured of a successful result.
Composition is not easy to get into, but once you have acquired a little experience, you will find that the enjoyment you derive will be much greater than any other form of improvised painting. Possibly for the first time, you will be really in control of what you are doing. Perhaps this is the reason why so many artists have elected to work away from nature. Because then they can be truly creative, like nature.