If one player starts the suit with red I do have a lower red but I have the black card too, could I use the black card to trump the red play or am I forced to play the red?
You must follow suit and play a red card (or you can always play a special card).
If I need to play the red, how would a black trump any other color?
If you don't have a particular suit, you can play any card. In your example, if you don't have a red card, you can play the trump suit.