Tartan is right - there really are no rules to this...Ok....to clarify....i guess there are some somewhere --- but truthfully - you could do it with 2 colors if you wanted to - or as many colors as you will have "trips" for the size quilt you want. I have one in 2" squares - all floral fabrics - that "fades" from dark to light to dark - all the "trips" are different fabrics. Do what is pleasing to you .... but i will say that a queen size of 1.5 inch squares will take A LOT of squares !!!! You might want to use the water color method of mounting your squares onto light weight fusible interfacing - then all you have to do is fold rows and sew straight lines. Good luck !!!