Joe,
The light brushmark is really just a central non pigmented area. It is like some of the other pix I posted showing lilies with white centers. It just so happens that this one doesn't extend as far out on the petals, so it looks like the shape of the normal brushmarks. After doing lots of crosses you will have brushmarks made from small spots that look like Tangos, Tangos that look like brushmarks, all sorts of center color flushed lilies. Some of it is predictable, but there will be surprises. I'm not surprised that this one turned up as the parents were a 1a pink with white center tango red spotted lily crossed with a 1a white with heavy dark brushmark. There is a third sibling seedling to these that is a 1a white heavily spotted with dark tango spots, but the form is not that nice.
So the answer is no, I wouldn't call it a freak mutation.
