Re: Why does my white linoleum floor turn yellow under kitchen nonskid mats?

My thinking [in the case of the rubberized rug backing] is that some of the 
antioxidants used in the latex backing compound are of the 'staining' type 
[which are generally amine- or sulfur-containing] and could easily migrate to 
the residual unsaturation in your linoleum.

Are you sure the floor is linoleum and not vinyl [PVC] ?

If the rug is imported, it is quite possibly backed with a natural rubber latex 
compound which will contain sulfur and a number of other staining moities as 
well.

If polypropylene is only the face fiber, I doubt that it is causing any 
staining of the floor.

Ken