
The brown tinging is quite common in Border Collies. Mine has it on her rear featherings and tinges of it on her body. She doesn't carry any red or tri colours.
It always looks worse in the summer as the sun tends to lighten it. The tinges on her body are glossy, but rear featherings are usually dull anyway.
Mine is three and I don't expect they will go now. I never noticed it in BC's before I saw it on mine, but looking around the show ring there are quite a few with it.