As you have correctly noticed, there are currently 3 editing features (i.e. Bold, Underline, Italicize). We are working hard to provide more features.
Meanwhile, a quick workaround is to first write and format your signature (including text color, font, hyperlinks, etc) in a word processor or browser. Then copy and paste that signature into Inky's signature editor.
Inky's signature editor can be found in Settings.