On Wednesday my daughter had lessons. The instructor and I noticed that the 3 horses that were being used all had a significant difference in tail color from 2 weeks ago. My daughter's (lesson) horse is a red roan, however his roan is fading with his winter coat, but his tail has always been a red color. The other 2 are sorrel with tails that have been an exact match to their coat until recently. Now all three horses have dark brown tails. One of them is so dark it's almost a purple color.
These three horses are kept in a pasture with a shelter because they're all tough keepers in a barn. My daughter's (she's the only one that rides him now) horse can't eat enough hay and alfalfa to keep weight on him in the barn, and the other two are still young and eat about 3 times what the average barn horse eats. They're in the hay field after a 2nd cutting (they do their own hay at this barn) and it's been really wet here so the grass is nice and green.
The instructor says she's seen the roan fade during the winter often, but never the tail changing so much. Could the nutrients in the grass be doing causing the tail to change so much. The manes are not changing at all. No, I don't have pictures. The lighting was all wrong on Wednesday and I couldn't get the settings right on the camera to show color. All you can see is my daughter's horse's bald face glowing. (His white is almost opaque now!)