Even if they can't see the color, they would still see something. There would be an object there, regardless of their ability to see determine its color. Humans don't see UV or infrared light, but that doesn't make an object that only reflects UV light invisible to us, it just appears black. A red object reflects wavelengths of light that humans perceive as red and absorbs all other colors. A yellow object reflects wavelengths that combine to appear yellow to us. If something reflects all light in the visible (to humans) spectrum it appears white, if it reflects no light and absorbs it all, it appears black. Thats part of the reason white things stay cooler in the sun and black things get hot quicker. Its also why a red light can be shined on a white object and the object will appear red, it only has the red light to reflect, but shine it on a black object and it stays black. So based on this, crappie can most definitely see the red hooks, the only question would be whether they actually appear red to them, or if they appear simply as black, which wouldn't be much different from using a standard bronze hook. I haven't ever seen a definitive answer as to the ability of fish to differentiate colors, but I feel like adding some red to flys/hooks/jigs can't hurt. If its in the right spot, (gills) then it will look natural to a fish, they either see red and it matches, or they don't see red, so it looks black and the same would be true of a natural fish.

TL;DR version: Bottom line, they see the hooks, whether they appear red or not is another question, but color perception doesn't make an object invisible. If he's seeing better results with red hooks, I'd chalk it up to the red being an extra attractor, but they certainly do not become invisible.