why do some flowers come out a different colour to the rest?

Depends which flower you are asking about

Which flowers are you talking about? If there are white stripes, it's usually got a virus.

its down to genetic combinations. a bit like skin pigment in humans. it can be very complicated. do you mean on the same flower spike or flowers on different plants.
welcome to GoY too.

i have a rose bush that flowers pink. one year it got all pink roses and a single white one. was very strange and has never happened again.

it could be some sort of reversion to the plant's natural colour before it was cultivated.