This using tunisian stitching, and this particular stitch is the Simple Stitch. Here is a good reference to how to do the tunisian simple stitch, and an overall excellent website in tutorials in many types of tunisian stitching: Tunisian Stitching
Base Row (+ return row): http://www.stitchdiva.com/custom.aspx?id=41
Yarn: I used worsted weight yarn from Vanna's Choice (the purple color "eggplant") and from Impeccables (yellow color "butterscotch")
Size Hook: I used J, and worked it loosely. Tunisian stitching I learned should be done loosely anyway!
"tss" = Tunisian Simple Stitch
forward row 1: base row (see above)
reverse row 1: return row
forward row 2: tss each stitch
reverse row 2: return row
repeat f/r row 1 and 2 until you are satisfied with the square for the first color. In my case, I stopped at 25 rows. You can make this more or less for your preferred look.
When changing colors, you change colors ON THE RETURN ROW.