Will someone please answer a few questions for me about my new angel fish?

I just bought 2 younger angelfish both about 1-1/4 inches tall one is black the other is white. well the white one goes all over my 20 gallon tank and explores it while the black one just cowers in the artificial plants by the filter intake. Is there something wrong with the black one and also how can i introduce them to artificial foods such as freeze-dried brine shrimp and flake foods??