I do have exactly the same problem and search for an answere but I couldn't find one since.
Here's my story:
Until a week ago I used to include a picture on top of every entry via custom fields. Then I began to use the post thumbnail function from wordpress. So I changed every entry and didn't use the custom fields anymore. It worked out pretty well, the colors were and still are all right. I also posted a new entry using the thumbnail function - but with the next entry everything went wrong. The pictures now look old and dead and do not have much saturation. If I click on the image in the entry it opens the fullview image and everything is alright. Or it also works if I include the image via html (But I really don't want to keep doing so). Here you can see the difference:
I've also tried to resize the images exactly the size it has to be so wordpress doesn't have to make it smaller. And that works, but I don't want to resize it because if I change the layout and need a bigger image I would have to upload all of them again. And of course my images are saved as RGB images.
So here are my questions, hoping someone can answere them:
Why does WordPress do that at all?
Does anyone have experienced this and found a way to change it?
And why do the pictures that were already uploaded all have their normal colors but the new uploaded ones do not?