I don't know why you haven't gotten any replies yet -- I'll try to help, but others in LE are much more experienced in the floral department, so hopefully they will correct or add to whatever I post.
The very few times that I made stringers for making irises, I used a home-made optic mold. The home-made mold just made it easier to get a fluted gather of glass to lay the contrasting color in to the flutes (between the ridges).
I tried something like periwinkle as the base with dark transparent amethyst for the stripes.
Another one was one of the dark opaque golden yellows with a transparent amber for the stripes.
Just look at photos of irises on the internet and match glass colors to what you like. I used the concept of opaque base glass with transparent contrasting stripes. (Using the transparent for the stripes gives more dimension and color shading as you lay down your petals.)