Like:
Blended Mixed
YELLOW ------------------------ORANGE--------------------RED
The closest I think you can get is to insert another note as opposed to putting the text on the line.
If you click on the line you will generate a new note that is in the line and then you can style the note in an appropriate way to your needs. Thus in your example click on the line, type Orange and don’t add any styling.
You can offset the text from the line by inserting “option-return” at the beginning or end of the line, but the text will still remain horizontal and there will be a space in the line taken up by the text box.
Yup, that’s the way to currently do it, simply double-click on your line and annotate it. You might consider creating a special “label” style that has a distinctive and perhaps more diminutive look to it, as well.
Note that we do have proper labels on the list of things for a future version (as well as line colours).