Colors in brush tips are not acknowledged by Harmony, as brush tips only use the colour you are painting with in Harmony.
To make your own brush tip, you must first create an image file containing the brush tip, either in Harmony or in a third party software, like Adobe Photoshop, then import it into Harmony. In the Brush or Eraser Properties dialog, you will find a set of default brush tips which are bundled with Harmony, but you can also create your own brush tips, then export your collection or import an existing set.