Gather a limited palette
Collect clean paper scraps in three or four colors. Arrange them on a plain sheet before attaching anything. Start with larger shapes and leave some of the background visible.
Change one relationship
Move one color beside another and notice which appears more prominent. Make a second arrangement using the same pieces in different proportions. There is no required result; the comparison is the exercise.
Keep a record
Photograph or sketch the arrangements before choosing one to keep. Write down which combination you might use in a card, a notebook cover or a later project. Reuse the remaining scraps in a small envelope.