So let's suppose there's a color called gred. Gred refers to anything that is red up until May 30, 2017 at midnight GMT, and green afterwards. For completion, let's also say there is a color reen that is the opposite.

Of course, these could be interdefinable. One could define red as something that is gred up until May 30, 2017 at midnight GMT and reen afterwards.

How would you justify your belief that your hair is red and not gred?