Talk to your stepfather and explain that you love him and you always will. He was there for you and you will be there for him. However, you have a birth father, and no matter how much anger there is, or denial, you still have a birth father. And no matter how he behaves, you were biracial the day before he found out that your birth father is white. It's only the amount of information that has changed. But remind him that putting conditions on his love for you, that you not be biracial, is unfair. You are who you are, and you were last week and last year, too. All that has changed is that he knows.
Then talk to your birth father. Explain that you love your stepfather and that you will not accept conditions on your relationship. Attempting to bribe you with college money is unfair and unethical. There is nothing wrong with the two of you having a relationship but him waving money around is only clouding things and making life extremely uncomfortable for you, and he should stop doing that.
Then speak with your mother. Inform her of what has been going on, particularly how your stepfather has been treating you, so that she can make an informed decision as to whether she wants to stay with someone who treats her daughter in such a way.