LogFAQs > #906843416

In the manga series Trunks and Bulma both had the same colored hair, and it was purple most of the time. Bulma was given blue hair for the anime adaptation, but Toei (for unknown reasons) kept Trunks' hair purple. Toriyama then decided to design Trunks with blue hair to match Bulma in the anime, but Toei had been changing his hair back to purple.

But for some reason, with his return in Super,Toei did not change Toriyama's character designs for the arc. So Toriyama designed Future Trunks' blue haired look, and it was not changed. This even leads to some characters acting like he had always had blue hair, as sort of a hair retcon for the character.

Though the change came about with production mishaps, fans have decided to come up with their own reason that sounds just as feasible. People's hair color changes when they get older, so fans have just theorized that this older Trunks now has hair closely resembling his mother.