Can’t understand why the oil paint brushes seem to be picking up black even in a color

I'm working with the newer Apple Pencil on the newer iPad mini and using the oil paint brushes and color.
This has been happening halfway through a painting.

I lay in a color and then when I go to use the dry brush feature, (magic button ) it seems to pick up a black from somewhere and make it predominant in the color mix. Could it be picking up some submerged sketch line, which I can't see and is no longer in another layer?
I try clearing brushes, changing brushes, changing direction, slant and size. it's frustrating.

And what am I missing? It's an enigma.