On my Samsung Note 8, photos are rendered with very different colors in email previews vs if you download then view them, the email preview shows the colors as being very blown out.
Additional note, this only happens with photos sent from an Iphone. :-D
All modern phones have some sort of friendly color space selector, this particular Samsung is set to be rather neutral. The interesting part is how dramatically different the preview in gmail is vs the photos app. My guess is Gmail is just doing something horribly wrong with the attached color profile in the photo.