[Mehve]

Personally, "coloured" feels like too strong of a word for the 650's, but they DO tweak the sound just a little. If you read through the appreciation thread, you'll see a lot of comments to the effect that they sound just a little relaxed compared to many headphones, and that they lack some of the bite that some people like for their rock/metal. When I compared them, I found my music (Prog Rock, Jazz, Trance) to be more enjoyable than with the 600's, although I'd probably concede that the 600's were more "accurate".
 
It's worth noting that if your positive experience with Sennheiser was with the PC350's, those sound NOTHING like the 650's do. The 350's are a lot more aggressive in the mid-range, and much lighter on the bass. Completely different animal, really.

 

[mudo]

I think that although HD650 are a bit laid back and relaxed you shoudn't be too scared about it: You still will hear treble and a detail. They are smooth, but they won't hide everything to achieve smoothness.
 
I found HD650 to be a beautiful balance that will probably please most ears that try them.

 

[High_Q]

HD650 are great headphones when amped properly.  I recommend you do some research on how to properly amp HD650.  What I like about the 650 is that high frequencies are not over emphesized like the k701 or my Beyer, the overly sharp pitch could get overwhelming after awhile.  They seem to have very realistic sound because the sound is not overly dramatized.