Color Fringing Seen When Using a 19 Degree Source Four at a 15 Foot Throw
"Color fringing" is the lens chromatic aberration.
"Chromatic aberration" is just a technical term describing the fact that different colors (wavelengths) of light bend by slightly different amounts going through glass. Blue light is bent more than red light. That's why a prism splits white light into a rainbow. In the field of the S4, away from the edge, this effect is washed out because you have a mix of colors that produces essentially "white" light. But at the edge, the red light is bent more toward the inside of the field (so it is washed out by other colors), but the blue light is bent toward the edge, and with no other colors to wash it out, you get a blue ring. Actually, if you walk up to the wall and look closely at the edge, you will see violet light outside the blue, and faint green light inside the blue. With a pattern, you will get a colored edge around every part of the projected image. With the 36 degree, this effect is reduced significantly (but is still present).
The reason it is reduced is because the 36º fixture is the only fixed focus unit with two lenses. The chromatic aberration caused by one lens is corrected by the other.
The only way to correct this would be by designing a lens that is composed of two different lenses, of different materials that bend colored light differently (different index of refraction), that are cemented together with some kind of optically neutral adhesive. Again, the chromatic aberration caused by the first lens is canceled by the second. This is called an achromat, for obvious reasons.
Unfortunately there is no easy "fix" for this problem".