Board Thread:Boruto Discussions/@comment-30830650-20170113060043/@comment-734582-20170119092620

Georgio722 wrote: Of course, there is also plot. It's not plot. If you google briefly you'd find out that Naruto had a 50/50 chance of being a redhead. Naruto is not a redhead but potentially a carrier. Hinata's family has no traces of red hair whatsoever, so chances is that Naruto's kids are genetically unable to become redheads. Alternatively, if Hinata does happen to be a carrier, it's a 25% chance to have a red headed child.

so TLDR: genetically their kids are unlikely to be red. It's not plot, it's science.