Do not get offended...please!?

I'm mixed I'm 1/2 black and 1/4 white and 1/4 native american. I really wanted to know how come if you're half black mixed with anything else your automitically black??? What about the other half of you? I don't understand...for example why is Halle Berry black...when she's half white? Why is Mariah Carey black if she's mainly Hispanic? I don't understand is there some kind of rule. I'm not trying to deny my black ancestry but deny the other ones either....why do I have to chose.