I love the colors in the notebook and I generally require students to color (remember NO magic markers...they bleed through to the next page). However I don't always have to time to let students color. In those cases I allow students to go back through their book, if they are done an assignment early, and catch up on their coloring. That is one of my "If you are done early" activites (along with read quietly, etc.).
Above are pictures of two venn diagram activities, one with color (last year) and one without (this year). The student who hasn't colored will most likely color both sides of that assignment during free time that she has.