I have had similar issues especially in the thumbnail.
The middle is the original.
Left is what the thumbnail upload looked like.
Right is me over correcting in order for the thumbnail to come out correct after uploading.
I probably would not recommend over correcting your pages, seeing how most people might not even notice the difference. I do understand the frustration of art coming out dull after uploading. But I think it's more an issue with how the internet and image processing functions. And I felt like Instagram and Facebook (both own by the same company) also messes with the color of uploaded images.