Well technically it DOES say in the rules. An upcard is the first card dealt in a new hand.
So if the color is determined by the upcard, then it is obvious that each new hand has a new upcard.
If a hand is a draw, there is no winner, and a new hand is dealt; therefore, new upcard.
Don't know what else the rules could say to make it plainer. Color is determined by the upcard.