Your child likes to wear shorts or Tshirts that have pictures of Elsa on them (see the picture).
Is it colloquial to say "You like to wear Elsa, don't you?" or "You are in Elsa now" the same way we say "You are wearing blue" or "You are in blue"?
Note: Elsa is the name of a character in the movie “Frozen”
It might make sense in context, but it isn't particularly idiomatic. It sounds like 'Elsa' is the name of the clothes label (as in "The Devil Wears Prada") and nobody really speaks like that outside of the fashion industry.
The most natural thing to say would be to use 'Elsa' as an attributive noun and refer to "your Elsa t-shirt", for example:
You like to wear your Elsa T-shirt, don't you?
"You like to wear Elsa (now), don't you?" is proper English and conveys that it is a (new, especially when you add "now") habit or trend.
"You are in Elsa now" could mean that you are only wearing Elsa-themed (or Elsa-branded) apparel on this one day (that the person sees you). This is (as Seeker suggests) a little awkward, and I would not say it like this. Instead: "(I see) you are wearing Elsa today."
"You are into Elsa (now)" means that the speaker assumes you are a fan (but not necessarily of clothing) - could be of listening to the soundtrack, watching the movies, wearing the clothes, using an Elsa lunchbox, or just talking about it. Add a question mark at the end to turn it into a question. But if you point to the shirt when you say it, or add "How many (of these) Elsa shirts do you own?" it becomes much more obvious you are talking about clothing.
"You are into Elsa (or Elsa-themed) shirts now", "You are into Elsa clothing now, aren't you?" or "You really like to wear Elsa shirts now, eh?" (eh? is really Canadian, but you could use "right" instead anywhere, or any number of other local variants) would narrow it down to clothes.
Similarly for blue - "You really are into blue", etc.
