Why Trunks Has Blue Hair?
Bulma Was Given Blue Hair for the Anime Adaptation, but Toei (For Unknown Reasons) Kept Trunks' Hair Purple. .. . So Toriyama Designed Future Trunks' Blue...
Bulma was given blue hair for the anime adaptation, but Toei (for unknown reasons) kept Trunks' hair purple. ... 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.
Why did they change Trunks hair color?
By the time of Battle of Gods, Akira Toriyama had begun coloring both Trunks & Bulma with blue hair– not purple. While Toei kept kid Trunks' hair purple, Future Trunks' hair was made blue to match his modern character design.
Why does Trunks look different in super?
Future Trunks' Hair Color
His hereditary hair color finally made him look like both his mom and his maternal grandfather since he didn't have the brunet hair of his father's genetic line. This actually makes him an outlier, as Saiyans usually have dark hair instead of blue/purple tresses.