@DIO:
Why does Future Trunks have Blue hair instead of his Purple Hair?
I doubt it's Super Saiyan Blue for one very simple reason: Goku's hair, when Super Saiyan Blue, stands up in it's "Super Saiyan" style rather than resting in the "Default" Haircut like it does in Super Saiyan God.
It stands to reason SSB Trunks would look like SS Trunks, only with Blue hair. But his hair is not in its Spiky SS form, it's resting like it does when he's base form.
I think I know why it's blue here.
In the Manga, Bulma's Hair is Purple. That's why Trunks' hair is purple, he takes after his mom. Even in the Manga, Trunks' Hair is also purple.
This makes sense and is consistent.
For whatever reason, the Anime staff decided to change Bulmas hair to Greenish Blue.
But… for reasons that astound me, never thought to change Trunks' hair to match, so the family resemblance is lost.
Enter the new movies and Super. they have Manga versions, but are clearly Toei's babies as it's obvious It's primarily an Anime product. That's why Toriyama's movie designs for Bulma for Resurrection F had the Anime Blue hair.
So clearly, he's doing these designs with the anime in mind.
So, with all that backstory out of the way, what are some valid reasons why Trunks' hair might be Blue in that pic?
1: Out of habit, he colored it the same as Bulma's hair, so he used the new color he's using for that.
2: This will be the first time Future Trunks and a Kid Trunks who is old enough to speak are gonna be in the same room together. Maybe he wanted to differentiate them more?
3: Maybe this isn't the future Trunks we know, but one from an alt universe altogether, and the hair color is a sign of this.
4: Toriyama just plain forgot either of their hair colors were purple. Remember, he forgets whole characters exist. Ask him who Lunch is.