mod_rewrite : redirecting URLs based on comparison of path components

I searched the web and read through several tutorials, but I can't make up a
working solution for the following problem. TIA to anyone that can help me out!

http://RED.domain.com/path1/BLU/path2 ...since BLU not equal RED then redirect to:
http://BLU.domain.com/path1/BLU/path2