A quick interim fix could be simply adding an exclamation mark on the end of the text when it reaches a red state.
Light Dehydration: Thirsty
Medium Dehydration: Thirsty!
Heavy Dehydration: Thirsty!!
This would make it easier for the colourblind people without giving an advantage to colourblind or normal players.
Moving towards beta, a more elegant solution could be developed.