Each yarn carrier 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 carriers that are currently in use in the design, are outlined with a dark outline. In this case, only the third yarn carrier is currently used in the design.
The yarn carrier that is currently selected for drawing is highlighted by a slightly darker background around the square button. In this case, yarn carrier 1 is selected for drawing.
When you're drawing, a yarn carrier is always selected, both in Pattern View as in Stitch View. In Stitch View however you will not directly see that you are drawing with a different color, so make sure to have the right yarn carrier selected!