I'm definitely no expert in genetics and stuff but I have been trying my best to learn!
I have a mixed flock of Silkies which I show. Many people that show don't have a mixed flock, they have seperate colour pens.
Since I'm on a really small scale (only 7 breeding silkies) having a mixed flock works best for me. My cockerel is also white so when I breed his chicks back to him, the majority of their chicks will end up white anyways. So I'm my case it works.
Like you, I was worried I would get mixed colours on the first generation of chicks. In my pen I have a pure white Cockerel and pullets which are partridge, white, gold and blue (all which have been bred pure).
When I hatched their chicks I ended up with fully partridge chicks from the partridge x white and pure white chicks from the whites x whites and then one white chick from the white x gold (the gold must potentially have a white parent somewhere in its history).
From what I can see, the chicks are not mixed colours. I believe in the first generation the chicks will be either one colour or another, not mixed colours. They shouldn't come out for example being grey with red patches etc. They'd just be fully grey.
The first generation chicks will hold the colour gene from both parents regardless of what colour it's physically showing. If you breed the first generation to a non related bird that has the same colouration (for example a first generation partridge bred to a non related pure bred partridge) you should get the colour you want.
However, if you breed the first generation back with their parents or siblings, you might end up with a mixture and chicks that have leakage in colour.
I am still learning myself but this is what I've picked up so far