很火=many people kown it；很红=many people like（or love) it；they are used as " very popular"

When “很红”and“很火” are used as " very popular ", they are just the same in meaning . But as a native speaker, I usually use “很红”to describe a person.

在我的印象里，“很火”一般用于非人的东西，火 = 红火，用于非人的事物。 “很红”一般用于“人”，“红”的意思可能来源于“走红”（Vi.）, "红人”（Noun），都用于人。

很红can means popular and very red, and 很火is just very popular. So when you are using both of them as 'very popular', they are just the same.

Actually, there's no big difference between them, and they all mean very popular.