The good news is this seems to be part of a known bug and there is a work around. The bad news is it's a small hassle to fix.
I started noticing color loss from iPhoto to WP, and it was driving me crazy. It seems to affect photos differently, and though I don't know see a clear pattern, the worst tended to be those I had adjusted in iPhoto, especially where I had increased saturation. The effect was pronounced fading and lots of pixel noise.
The workaround actually comes from another thread on the subject of animated gifs-- but I guessed that this might be an extension of the same issue, and so it is.
You need to circumvent the WP resizing functions by doing it beforehand in Photoshop, etc.--but that alone will not fix the problem.
Once you've resized, uploaded, and inserted your photo to the post (having selected the "Full Size" option), you need to click the "Edit Image" icon (left corner), then go to the "Advanced Settings" tab and delete the "Width" and "Height" values in the "Size" row. Make sure the fields are empty (not "0"), then click "Update", and you should be good to go.
Now the only problem is I've got a masters thesis due in three weeks, and so many bleached out posts begging to be fixed...
Hope that does it for you. Cheers!