Now that I always point to the Material Style Guidelines, the designer demands me to change the floating color on TextInputLayout when showing error messages, like in the guide lines (scroll down to "Error message"):
It seems to me that this is impossible, but I may be missing something. The method setHintTextAppearance() will only change the collapsed hint, while the floating label is drawn privately in CollapsingTextHelper.
EDIT
The following question mentioned in Anuj Kumar's comment WOULD help, but it seems this ticket "fixed" it, so it doesn't work anymore. I tried it with v25 and v27 of the design library in a simple project, but it doesn't work. I need another solution.