Oh sorry, I realize now I didn't read your first question properly. If it's det, it will have to be rött too, because the adjective changes with the gender. And I see in the incubator that the solution Det är rött på insidan is already accepted.
Since there is nothing to indicate what exactly is 'red' .... I thought that using "Det" is just more common. Am I wrong? I know to use 'det' for 'ett' words and 'den' for 'en' words, but the noun is not known in this case ... it's only 'it.'