Sceloporus sp. have brighter colors when looking for mates. Also males are more colorful then females outside of the breeding season. So your observation could have several possible implications. Consider the following:

1. You are seeing more females then males. The could be the result of pestesides or other polution. This very same effect has been noted in human populations in some locations.

2. you are observing shorter breeding seasons. Temperature related?

3. Natural selection, is there a new predator selecting the more colorful lizards? Human collection?

I would eliminate food as a possible cause given that captive bred Sceloporus sp. are just as colorful as wild caught given a diet of only crickets.