You can use whatever you want! Some people make them with only a few colors. Personally, I like to use lots of different prints in a particular color scheme and keep the center square the same because it forms a secondary pattern when I cut/sew them back together after flipping the opposite ones. I also tend to keep the same parts similar, such as all the outer corners dark, inner squares light, etc. I like the way it looks. I've done a few. One was black and white with turquoise. One blues/yellows/greens with teal centers. One of safari fabrics that I used squares from a panel for the large squares. I'll see if I can locate the pics for you.