VETSCOPE

My pet's skin color has gotten very dark since the beginning of this year (it used to be a silvery color but has gotten even darker). What do you think might be the cause? Is it the change in hormones because of neutering? Is it because of the ultraviolet rays since she likes being in the sun? Or is it just aging? She looks healthy so I guess I don't need to visit the vet, but I'd like to know since it's been bothering me.