purple marks on linoleum

Ive been in my house for almost 3 years.When i moved in the floor was yellowy with purple marks under a rug on the linoleum floor in my kitchen.Since I've been here the marks have spread beyond where the rug covers and have gotten darker.The floor is linoleum,in front of the patio doors,there is a heat register there too and I have a rug over them.Does anyone know if that is the linoleum turning because of the sun&rug or could it be mold?I am planning to redo the floor but I want to make sure if there is a problem(like mold)that I fix that first.The under neath of the florring appears to be dry and free of marks.Please Help?

 

PVC migration from rubber and vinyl backed rugs can cause yellowing and discoloration of vinyl floor covering. Purple discoloration is usually a sign of moisture beneath vinyl where there is mold and mildew. Heat and sunlight can also cause yellowing and discoloration of vinyl flooring. Make sure seam between vinyl and door is sealed with silicone caulk to prevent moisture from entering. Discoloration of vinyl is permanent.