Online tutorials show artists making a stroke in one color and then a stroke in another color and then WITHOUT going to the color picker, they press where the two colors meet and get a midtone of the two. I can get it to work if I go from the oil brush to the color picker and then back. I can't get it to work on my IPAD pro or my mini iPad. Any ideas.

Thought another way to do it would be to double tap on my Apple Pencil but that doesn't seem to work either. HELP!