Part of central incisor changing color and becoming more yellow/brown and gray/translucent. What is the cause? (Photo)

Hi:From the photo it appears that there is staining going on.  Most likely from either black or green tea.  If it is stain, it should be polished off rather easily.  The remedy if this is the case is not not doing out of an open cup, use a vented lid cup such as the ones in starbaucks.Best,Dr. Maddahi

The good news is that it's just superficial staining and can be easily removed by your dentist. Sometimes a slight microbial change in your mouth can lead to this kind of stain. 

Have you EVER had an injury to these teeth from an accident, sports, punch or blow to the chin that may have slammed your lower jaw into your upper teeth?  Have you recently started taking Iron of vitamins with iron in them?  Have you started a new habit of drinking wine, tea or coffee?  You should go visit a dentist to have this checked out.

Based on photo and description you provided it shows that the teeth around the one you are concerned with have also some areas that appear grayish and those areas are where we most of the time see natural translucency being a factor. I believe that one of the reasons you might be having an increase in grayish appearance is because the enamel of your incisor has worn off and became even more translucent due to a possible attrition from the opposing lower tooth. Have your doctor looking at it next time you see him/her