I have not had that problem before but maybe could it be an issue cmyk images being converted to rgb format? Do the exported images of your art on your computer have the vibrant orange color or yellow green color?

I export my artwork to RBG color profile, and as for how they look as a png on my computer, they look slightly less vibrant than in my art program. But I think it is an issue with FurAffinity, because after I upload my artwork, it looks much more desaturated in the preview box on the finalize screen, it must be an issue with them changing the color profile to a different one :/ Although when I proceed with finalizing, the image does gain back some color, but not to the same vibrancy. It becomes more vibrant than as a png on my computer, but less than in my art program???

All of this could just be me being not knowledgeable enough on this sorta thing, because I only learned recently that I could even change the color profile in my art program.