I have found that light colored tulle washes away definition and pales and blurs lines. However, a single veil of black or navy tulle seems too enhance color richness. Perhaps changing tulle color will provide you with the answer to your dilemma. I'm not sure how a portrait quilt is constructed. With my art quilts, I "color" with fabric scraps that are glued between the lines of my design sketched on background fabric. The tulle holds all in place once quilted. Your portrait is incredible!