If you add a tag to a passage (near the top of the passage), you can then select a color for that tag (using the down arrow on the tag), and the color will be visible on the passage nodes with that tag. If you have multiple tags with different colors on a passage, then the node will show all of those colors. That said, there are only six colors to work with (or seven, if you count "no color" as a color).
Hope that helps! :-)