How do we know what color dinosaurs were?

Pretty simple question. I've seen videos about certain dinosaurs with feathers and people have spoken about their iridescent colors, but I'm wondering how we know they were iridescent? How do we know what colors their feathers were?

I'm making a big assumption here that there's some scientific process by which we've figured this out, rather than just looking at fossils - because I'd assume none of the fossils are colored anymore?