If you work with line breed fish then there are simple rules.
ie pingu guppies geneticaly have very little colour gene's.
Cross and fancy coloured male with a pingu female and then seperate all the female fry and rear alone to keep them vergin.
You then cross the females back with the dad to hopefuly fix the colors.
Working with shop fish which have been breed with mant males are difficualt and you could wait a long time to find something u like.
but it's fun and I've tried many times myself.
Helter