We see this a lot when cleaning tile and grout. What sometimes happens is when the installers are done with the tile they use a sealer to keep dirt out etc. According to certain companies you are supposed to re-apply this every 3 years or so. What I am getting at is those color differentiation can be from areas they missed when sealing, or places where the seal has worn out. Sometimes we have seen people use acidic cleaners which strip the sealant as well which will cause that color differentiation.