Saiyans always have black hair with certain characteristics. That's why Goku and Vegeta always had the same hair. This does not apply to the Saiyan funds, however, and this truth was known in Dragon Ball Z on the occasion of the arrival of Trunks of the Future in the main timeline.
Both in the Dragon Ball anime and the manga Trunks shows up with purple hair. In Dragon Ball Super, Future Trunks 'hair changes and turns blue like Bulma's, while Baby Trunks' hair remains a more purple color. Why does Trunks have blue hair instead of purple and what color should he be in Dragon Ball Super according to Akira Toriyama's original ideas?
Let's start with an assumption: Trunks has mommy's hair the same color. This means that he should have them blue in the anime while he has them purple in Dragon Ball Z and only in Dragon Ball Super are they recolored that way. The flaw arises from the designs Akira Toriyama sent to Toei Animation's production.
In the Dragon Ball manga, Bulma has purple hair and, according to Toriyama, this is the woman's official color, although we're all used to seeing her in blue by now. Hence for the mangaka Trunks had to have purple hair and then when he sent the color for the future character to Toei Animation, they formalized the purple color for the anime, probably without knowing the relationship between the mother and son's hair.
Only with Dragon Ball Super, Toei Animation decided to recalibrate the situation and make Future Trunks' hair look exactly like Bulma's in the anime, but inexplicably leave the kid Trunks, who continues to have purple hair, out of the question. A similar error occurred with Vegeta at the beginning of Dragon Ball Z.