changing color on print - Flexi DTF Pro 22

I need to change a color on the fly and I know it can be done but for the life of me cannot figure out how.

Printing DTF white, so I need to change the white to 1% yellow so that the printer knows to put white under.  I bring up color mixer after changing the PNG to CMYK.  I cannot select the parts that I want to change.  I have done this in Flexi 21.