I'm not a geneticist nor a scientist. My knowledge of iris pigmentation is sketchy, but I do know that iris pigments fall into 2 different categories: carotenoid or anthocyanin. The carotenoids are the yellow, pink and orange colors and the anthocyanins are the blues and purples. The carotenoids are oil soluble and the anthocyanins are water soluble. Iris petals are made up of different layers. Just as an oil soluble paint and a water soluble paint will not mix, I don't think you will find both types of pigments in the same layer---I wouldn't swear to that, but I think it's correct. Since you can't mix the different pigments to get the colors you want, you can only achieve the effect you want by mixing different layers of the pigments--for instance, yellow (carotenoid) and blue (anthocyanin) to get green. There is a green pigment in plants (chlorophyl) but I don't think it is ever present in the petals.
So if you take Marvin's seedling and look at the crests, what you see is a blue with turquoise veins running through it. If you took a sheet of blue mylar and a second sheet of clear mylar and drew yellow or green veins with a yellow or green highlighter on the clear sheet, then overlaid it with the blue sheet and held it up to a strong light source, you would probably see much the same effect--a field of blue with turquoise or green veins running through it. Supposedly, the same process is behind the red effect we see in "red" irises, when red pigment does not exist in irises. So what you are seeing in the seedling may be an effect of two pigments interacting with each other instead of a true pigment itself.
On the other hand, there is a greenish blue pigment (delphinidin) which is present in certain
flowers which presents a near turquoise color at least. Delphiniums for one, which is where the pigment takes it name. Whether it is present in irises, I don't know--probably. I do know
that many times when you have a vase of blue, or even dark purple irises, and they start to fade, they often drip pigment as they go. Many of you I'm sure have observed this and if it
falls on a napkin or piece of paper, the pigments will run and separate and you will see definite blue-green color on the paper, as well as blue and purple, so I would think that blue-green pigment must exist in those irises. So whether the turquoise in Marvin's seedling is an effect of layering of yellow and blue, or is an expression of the concentration of a true blue green pigment, I couldn't say. It could even be both.
As a side note, people bemoan the fact that there is no true red iris. Believe me, I would love to see one, but I think we should all take comfort in the fact that we have such wonderful blues in irises. There are many, many plants that produce true red flowers; there are very few that produce the beautiful blues that irises do. Blue is the rarest color in the plant world, not red.
