Any idea why?  Tried bouncing to clip and it's still dark. 

Can you explain in a little more detail please.
 
It would also be useful to take a few screen dumps & upload them to  a site which provides you with the link codes so you can post them here.

Jonesey, when I have 3 or more instruments (ez drummer, SI bass, East West etc.) open in the PRV and I copy and paste midi notes as I move further into the piece, the PRV screen gets increasingly darker. With each new midi group that I paste, the white screen becomes a darker pink.   I've been living with it for some time but now that I'm adding more instruments it's getting harder to distinguish note colors. 

Turn off 'Show Clip Outlines'?

That's really odd. "I've never seen anything quite like it" [HAL-9000]
 
Really, try to post an image somewhere. I'd love to see the sequence as you add more tracks

That was it Adrian.  Thanks.

Good solution.
 
I never knew it did that in the PRV.
 
The only difference is, mine turn grey as opposed to pink!

Could be grey Jonesey.  I'm a wee bit color blind:)