Use distilled water to top it off.
Also, you might have coolant in there, but it's almost impossible to see. Best method to check:
Remove right side fairing, unscrew radiator overfill cap, insert a zip tie into the coolant jug (use it as a dipstick), pull it out and see if there's liquid on it. I'll bet there is....
Having the top of the fairing cut out makes it easy to shine a light in there, which makes it easier to see the coolant level from the other side, but some purple dye would really help (or, duh, a totally clear plastic overfill tank!).