all fabrics will fade from sunlight- not much you can do about that other than keep them out of direct sunlight-
it does not happen immediately- it takes time- but you will notice fading (and not all over-all at once-it will be -blotchy fading)
that is why you are recommended to keep your folded fabrics out of sunlight- otherwise the folds/areas that get the sun will fade-
you meantioned rinsing- did you also heat set? heat setting helps the dye stay in longer-
and it's been my experience that food coloring looses its color the fastest-fades quick---i suppose because it is made to be used in food- not on fabric so does not have the same (chemical composition) as textile dyes.