If I remember correctly, they excrete liquids/solids from the same orifice, which is why it is usually white...
Found more info on why it looks white:
Strictly speaking, bird poop itself is not white – it is in fact the urine element of a bird’s waste that gives it this distinctive color. Birds do not have a bladder that functions in the same way as a mammal’s does, and are adapted to make themselves as light as possible to enable the maximum flight efficiency at all times.