Are you using sRGB to export the photos?
What are you using to view the downloaded photos?
May i know are you using Lightroom or Photoshop. Also when you export the image before uploading to the Dropbox, how does the image looks.
i´m using prophoto, in this case I used an Ipad. But I have the same result on several mobile devices.
I use lightroom to export pics when I´m done with them. And the image looks fine before I upload to Dropbox.
Try exporting as sRGB, check the exported file before you put it in Dropbox to see if the colors are correct.
Ipads and other mobile devices may or may not be properly calibrated to show the same colors as you see on your computer screen.
If that doesn't work, I'd re-calibrate your computer monitor and, if possible, calibrate the screens on your mobile devices.
Nice! sRGB seemed to fix it. I was aware of mobile devices were not calibrated, but not that sRGB would "overcome" that problem. Thanks a lot, that saved me from a lot of pain