It is the most asked and awaited question that why does Trunk’s hair color changes from purple to blue. If we get back to the Bulma, in the manga series the hairs of Bulma were purple but later when it got transferred to the anime version, producers mistook it with the blue and this was the reason that they made Trunk’s hair as of blue color while in the Dragonball it was purple.
This is the same thing which happens with the Future trunks which have been introduced in new Dragonball Super. As the mistake held by the studio in case of Bulma so they decided to make hair color as purple for Trunks in the Dragonball Z and everyone accepted that hair color as it although looks nice.
Now, if we talk about the Dragonball Super, everything got redesigned by the Toriyama and they gave Trunks blue hair for this new concept to make them look like Bulma’s son and this is the reason due to which future trunks are given blue color by the Toei animation studios by considering the design given by Toriyama. So, this is the whole story which took place behind the color of Trunks Hair as everything started from an accident and now for giving that accident a generic view future trunks has given blue hair so they may look alike of Bulma’s which is pretty well.
So, everything happens to match the anime trunks with Bulma trunks by giving future trunks as blue colored hair for making a situation look alike as it is describing like Future Trunks always had the blue color and for justifying it some flashbacks has also been generated for the clarification.