Hi Stargazer. Well! Sounds like you need to do some cleansing! According to Traditional Chinese Medicine the middle of the tongue represents digestion/stomach/spleen and the sides represent the liver and gallbladder. Your tip, you said, is clear...and that represents the heart.
Here is a graphic tongue page: http://www.beyondwellbeing.com/html/tongueform.htm (warning: graphic tongue pictures! to compare and contrast...Interestingly I've always wondered about the spots on my boyfriends right side of his tongue...when I opened to the migraine page it had HIS tongue represented there! It's time for him to do a Liver Flush I see...)
Anyway, Stargazer...I'm glad that you've been tested for all sorts of things and you appear healthy as far as western medicine goes. With that out of the way, now you can take your health in your own hands and start cleansing your bowels to get your digestion healthy and then do some flushes to clear our your liver/gallbladder. Your tongue will be clear in no time, I bet!
Please keep us posted Stargazer...I hope your tongue turns pink soon....