If you want something to blend in well with the background just like Rousseau was famous for, you will need to restrict yourself to using just a few colours.
When we paint as children, we usually use just the colours that we are given on our palette, but as we grow up we will always mix them. Even if we have a green, for example, we will alter it slightly by adding other colours to it. It is not often that paint will be used just as it appears in the box or tube.