Plan ahead so that you can integrate the shift in background colors with the foreground. Your largest task will be to integrate both the color changes and the pattern changes to create a cohesive composition. By interrupting the pattern you will be creating a one unit composition instead of a design that infinitely covers the plane. In a complex design, it's easier to paint motifs onto a colored background than to paint around them. Light colors on a dark ground need to be painted twice.
Light colors can also run and make a mess.
If you encounter this problems, paint your background around the motif. It's a good idea to test out your colors before deciding which way to go with the ground. Mix the other colors and apply them to the colored grounds to see if light colors will go over the ground, or whatever you need to see. Remember that gouache will dry to a darker, more intense color than when it is wet. A dark colored ground may require additions of white for light colors.