I have been reading Eugene Hecht's Optics and the quoted text at the end of this post intrigues and confuses me. From his explanation, it seems to be that as the concentration of a dye increases, the reflected light switches to being complementary to the color of the dye. Question 1.) Do painters have to worry about the concentration of a dye? If the concentration is too high then the blue sky would reflect red-green instead of blue. This might seem problematic to other applications such as printing color images and food coloring. In most cases it seems to be the solvent is intended to dry out, and the dye will become concentrated in the end. The quoted text is as follows from Hecht Optics 4th ed. "A bottle of ordinary blue ink looks blue in either reflected or transmitted light. But if the ink is painted on a glass slide and the solvent evaporates, something rather interesting happens. The concentrated pigment absorbs so effectively that it preferentially reflects at the resonant frequency, and we are back to the idea that a strong absorber (larger nf) is a strong reflector. Thus, concentrated blue-green ink reflects red, whereas red-blue ink reflects green. Try it with a felt marker (overhead projector pens are best), but you must use reflected light, being careful not to inundate the sample with unwanted light from below. The most convenient way to accomplish that is to put colored ink onto a black surface that isn't very absorbant. For example, smear red ink over a black area on a glossy printed page (or better yet, on a black piece of plastic) and it will glow green in reflected light. Gentian violet, which you can buy in any drugstore, works beautifully. Put some on a glass slide and let it dry in a thick coat. Examine both the reflected and transmitted light—they will be complementary." Thanks in advance for any contributions to this discussion.