So I know this would be in no way intentional, but do you think SSJ4 Gogeta is red because he uses God Ki? Like, he hit some unspoken limit to how much ki a mortal body can handle and switched either automatically or on instinct?
Otherwise, why is he red? It's always bugged me.

DBH made pretty clear that only Xeno Trunks has God ki, everyone else didn't even know SSG existed until Chronoa told them.

I do remember how Hyourinjutsu's Manga!Super Goku vs. GT Goku animation had the latter speculate the same idea, that SSJ4 Gogeta had god ki, but he dismissed the idea after considering the prospect that others would've sensed Gogeta's ki, which they wouldn't have if he had god ki. Plus, with Heroes, they've since turned SSJ4 Gogeta's whole sparkling thing into its own form, so you can't even imply that's god ki anymore.

As to why he's red, or why any SSJ4 user is reddish, according to then-character designer Nakatsuru Katsuyoshi, the reason for red (at least when talking about it in the context of Goku) is because "it pretty much just seemed strong; I place a lot of importance on those sorts of impressions (laughs)." Further more, he has mentioned that the hair and eyeliner of the design was inspired by kabuki, the latter specifically by kabuki stage makeup, Kumadori, of which red makeup is typically used for the portrayal of strong, bold, and virtuous hero roles.

Also, on God Trunks, they have gone on record to admit that one of the reasons why they gave him Super Saiyan God is because they couldn't decide on what his fur color would be if he went Super Saiyan 4, which is hilarious to think about.

Damian0358 said:

Also, on God Trunks, they have gone on record to admit that one of the reasons why they gave him Super Saiyan God is because they couldn't decide on what his fur color would be if he went Super Saiyan 4, which is hilarious to think about.

Actually, it wasn't fur color, it was hair color. They couldn't decide if Trunks would have black or lavender hair as a Super Saiyan 4.