Each yarn feeder is represented by one of the numbered square buttons at the bottom of the toolbar on the left-hand side of the design screen, as represented in the picture below.
The yarn feeders that are currently in use in the design, are outlined with a dark outline. In this case, only the third yarn feeder is currently used in the design.
The yarn feeder that is currently selected for drawing is highlighted by a slightly darker background around the square button. In this case yarn feeder 1 is selected for drawing.