The multi-vitamin thing does have some effect, though how much is up for question. I raised a clutch of Sinaloans on bird vitamin-laden mice, and the reds on those snakes were quite stunning. It does make at least some difference.

Moisture works too. Milks kept in cages with moist hides tend to show better color red than dry skin. Soaking a milk before putting it on display helps with the wow factor.