Honestly, a lot of people probably couldn't tell you what color Tony's eyes or RDJ's eyes or anyone's eyes were and don't particularly care (some of them are colorists working for Marvel - I've seen panels where the colorist forget to make Gambit's eyes red & black, and his eye color is a plot point). I went through the Sirius Black blue/grey vs. brown eye color arguments once upon a time, so I notice those things.
I think it's a fannish culture thing, too. I started out in HP fandom, where if details differed between the original source canon and the movie, book canon always won (Harry always had green eyes regardless of what color Dan Radcliff's were, Hermione had bushy hair even though her hair in the movies was straight, etc.) so it's always jarring to see Tony's eyes described as brown even in 100% movie-verse fic. It's like the way I have to do a search and replace on Mag7 fic anytime Vin's horse isn't named "Peso" (which is even more silly than my eye color hang-up, because Peso and Chaucer's name aren't even canon and yet my brain insists all other horse names are wrong).
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I think it's a fannish culture thing, too. I started out in HP fandom, where if details differed between the original source canon and the movie, book canon always won (Harry always had green eyes regardless of what color Dan Radcliff's were, Hermione had bushy hair even though her hair in the movies was straight, etc.) so it's always jarring to see Tony's eyes described as brown even in 100% movie-verse fic. It's like the way I have to do a search and replace on Mag7 fic anytime Vin's horse isn't named "Peso" (which is even more silly than my eye color hang-up, because Peso and Chaucer's name aren't even canon and yet my brain insists all other horse names are wrong).