Hi, all--
I'm having this issue as well. The causes/solutions mentioned so far do not seem to be related, so let me add a few details.
First, the problem is intermittent (at least in my case)--it happens once every few weeks.
I'm in Safari 4.0.2 (OSX 10.5.7). The few times this has happened, I've had a number of sites open in tabs.
The sites open just fine, and if I don't scroll they remain normal. As soon as I scroll, however, the colours immediately desaturate. They don't go completely greyscale, but almost.
This applies only to the colours of the web site--not the entire monitor. No other application is affected, nor is the desktop. Even Safari's frame elements are unaffected. Only the content of the web site desaturates.
Here's the really mystifying bit: It's not even the entire content. At the bottom of the screen, in the bottom few centimeters, the colour fades from desaturated back to normal!
I was going to include a screen shot to illustrate what I'm describing, but like I said it's intermittent. It was happening half an hour ago, but I can't replicate it now. I have not restarted Safari, but I might be working in a new window (which suggests that closing and reopening the window might be a workaround--I'll try that next time it happens).
This might be one of the most fascinating and enigmatic (and, fortunately, pretty harmless) bugs I've run into in 30 years of computer use. Really curious for an explanation!