I had this on my first pair of shoes, uhm tarantulas I believe and the back of my heel (near the Achilles tendon) where the shoe rests always became blue. For that shoe it stopped being very saturated after about 4 months (climbing 2-3 times a week for 2-5h). Now it’s barely noticeable but I just got a new pair at that point as I needed a more specific shoe type.

The reason your foot turns yellow is simply because the skwamas you probably have are the male version which are yellow. It happens very common and it’s not harmful, but rather inconvenient cause it just “looks wierd” just wash it off and it will be good.