I was having the same problems, and after trying a few things, I think the issue is the following:
My guess is Dropbox saves a thumbnail/preview in some color space that is more limited than sRBG (or lower resolution, etc). This would enable them to limit bandwidth on any user just trying to "preview" the images. If you click on the preview link and save that image to your drive, you are only saving a copy of that thumbnail from dropbox, and thus lowering the quality of your photo. However, if you "download" the file to your disk, it takes a few more seconds (thus proving that the preview is a smaller file) and once you open the file you just downloaded to your disk, the photo pops up again in the right colors. I can't confirm this, but it sounds like what may be going on. In my case I have no issues if I open the files once I've downloaded them to my computer, but all the previews that appera from just clicking on the files that are listed in Dropbox in my web browser render horrible colors.
Hope this helps
Christian