Images have different colors at different sizes

Hello!
Not sure if you were able to resolve the issue or not, however my best advice would be to convert your images before uploading them to WordPress. I have found that it helps a lot to make sure the size I save the image as in adobe/gimp/etc is the maximum size allotted for pictures by your theme. Another thing I heard helps a lot is to convert the images to an sRGB prior to uploading.
Hope this helps if you haven’t already fixed the problem!