there are a number of ways to achieve a rainbow of colors & raggys tend to be pretty easy quilts- if you don't want to do it the way raggiemom suggested another way would be to choose how many fabrics/colors you are using for your 'rainbow'---say 9 as an example--
cut your squares, depending on size- and finished size wanted---an example would be a 9 x 11 layout==
you would cut 11 squares of each color- for the top- cut squares & batting squares-
number your colors 1-9
start layout with them in order- 1 thru 9
second row layout 2-9+1
third row layout 3-9 + 1-2
ect. you wind up with the colors in diagonal rows down across the quilt.
or- just cut your squares- make your sandwiches & play around on the floor (or on a bed) moving them around until you like what you see.