Color of tongue

The back of my tongue is a little white, whitish, or yellow, is that normal? Also, it get colored easily by most drinks I have (black tea, coffee, soda, berry, etc) then I have a hard time to clean and I need almost 10 min of cleaning. I have visited two family doctors who said this is a normal tongue, and tested negative for oral thrush on lab. Then I decided to visit a dentist who said this is not a thrush because the white stuff does not come off, soft (not hard) and, not hurting. However,  he thinks my filliform papillae are a little longer and that's why they look whitish and get colored easily.
Also, most of time I see a teethmarks on the sides of my tongue.
I have attached some pictures that are similar to my tongue
Let me know what do you think??

Source of image
http://www.doctorspiller.com/circumvallate_papillae.htm