Brownish yellow Rocks

[molotov]

So I've been noticing more and more lately that my rocks are turning a brown/yellow colour. I was expecting more of a purple colour. My tank is about 3-4 months old. The rock I chose to use was macro rock. I also purchased live rock off a fellow canreefer to seed the macro rock. Everything in the tank is happy and healthy. All paramaters are in check as well.

I guess my question is this. Is it normal for my rocks to turn this colour?