Of course all of the above are great suggestions. I try to be sure I use an odd number of colors--artists will tell you that 3 is a good color.
Our guild says to put one in that is a "sizzle" color; something you don't expect.
I also go by color families; all of the shades of one color, in an odd-no. amt.
No doubt there are many more great ideas!
Mariah.