Here is an easy test for color fastness -- and color absorbtion. Wet a piece of the dark color and a piece of the light color, place the light color on a white paper towel not quite in the center. Place the dark so it is both on the light and on the paper towel, put a weight on top (like a glass jar or drinking glass full of water) so it puts weitht on the dark fabric where it is on the light and on the paper towel. Leave it set over night. The pressure over both fabrics simulates the pressure of the seam and will let you know if the dark bleeds and if the light will absorb the dye. The pressure on the dark and the paper towel tells you if the darke will bleed. Saw this on a quilt show and it does help decide to wash or not (before I had to wash everything).