I have done something similar, sewing ecru stars on a blue field--Betsy Ross Flag Afghan. I use sewing thread the same ecru color as the stars and really anchor the stitches well to the stars, and if you are somewhat careful, it will not show through on the other side as the crochet stitches are so bulky. I was surprised how easy it was after all my worrying. I don't think it would work as well with a knitted afghan.