I think I see what it is, or at least part of it. I pulled petals from
parents, grandparents, and cousins to see if anything was showing up in
them. The odd patterned seedling is "cloud shadows" X (Imm x AMITY ESTATE)

The veins in "cloud shadows" are more or less the same color (near as I
can tell - at least they are violet, maybe slightly different shade or
intensity) from the throat all the way to the edge of the petal, but the
color between the veins is darker than the veins. So it has light
colored veins on a darker ground. I've seen some of the same in other
IMM X Csong descendants.

So that's part of it.

A lot of blooms have brown veins in the throat that switch to yellow
outside a white patch around the beard (i.e., anthocyanin drops out).
So maybe the veins had a lot of yellow with just a tint of anthocyanin
giving them a brownish tint, then where the yellow drops out, the veins
are very pale in contrast to the pigment that starts between the veins.
& the rest of it comes from just the usual white that so many hafty
irises have around the beard.

But I do wonder about those light veins in "cloud shadows". Not unique,
but somewhat uncommon.

Linda Mann east TN USA zone 7, end of bloom season and too hot and muggy
to be outdoors this time of day..