genetics - How can co-dominant alleles work in flower petals?

I am studying gene inheritance, and I understand complete dominance/co-dominance and incomplete dominant penetrance in flowers. I know how to recognize each case.

However, how does co-dominance work? If I understand correctly: when a gene is transcribed, both alleles are transcribed. Hence, for flower petals, if I have a "red" allele and "white" allele, and there is incomplete dominant penetrance, I get rose petals. However, if both alleles are co-dominant, why are petals either red or white? Aren't both alleles transcribed? Or perhaps, in the case of co-dominance, heterochromatin is responsible for shutting down one allele in lineage cells?

I hope that I am expressing myself correctly. Thank you very much for your time!