Like lots of plants, growing from seed might not be stable and so might not run true. The alternative is to grow from cuttings. Plant viruses can also cause this variation.

I have several that produce a paler first flower then deeper ones after. Heat can also play a part in colour development.
Wylie is the member to ask really as he breeds them.

Thanks for the advice both. We've certainly had heat this year and plenty of it. I can't message Wylie, not been able to access messages on here since last year I'm afraid.