Mobile UX Design: User Errors
Nick Babich

As a user, I am really not in favor of showing error messages, while I am still filling a form. For instance, what if a field is mandatory? If I focus another field, it will be highlighted in red, which I think is rude :) It’s like interrupting someone — they could fix their “mistake”, if only you let them finish their sentence.

Therefore I think it is better to only highlight the problems when submitting the form.