I don't know. caps? (oh right, we hate Polos around here, and it's a not a WG so this won't matter ) rejuvenator? maybe the tube's getting weak?
brightness isn't going to do anything with colors, brightness only adjusts the black level, in which case you want your brightness turned up just high enough to where a black background is not really visible in the raster, you want it to blend in.
you'll have to tinker with the contrast first (turn it as high as it goes before you get smearing) or if that doesn't get it, you're gonna have to start playing with the drive/bias pots on the neckboard (not a fun task).
if you can't adjust it out with the pots, then it's more than likely your tube going bad.