I love reading everyones tips on breeding for mutations... literally nobody knows 100% and i've noticed the less you know the more ridiculous your 'method' is
Here is the 100% truth.
It's conplelty random
You can get a mutation first breed or the 100th breeding breading parents and kids and or siblings with siblings have no affect on it.
I mutated my blue argy by breeding.. Mummy and daddy then bred the daughter with daddy got a son so mated her with her son then Darcy popped out with a full mutation of pearlescent blue/purple and white colour which looks like it changes in the sunlight