Can notehead colors be different colors in the same chord?
Yes. Enter the first note as usual – this will determine the stem colour, and then add successive notes using Control/Enter each time having set the notehead colour from the "Item Color" menu.
Alernately, to change the color of an existing notehead, position your hand-pointer directly over the notehead in question, then right-click and choose a new color from the Color submenu.
Edit: If you want to change the stem color after creating a note, highlight the note and do Properties. The Item Color on the Visibility tab will set the color of the stem, while the Notehead Color on the main tab will allow you to set the color of all noteheads for the note/chord. (This property seems to show the wrong notehead color sometimes for a single note if you have changed the notehead color via right-clicking.)
Thank you both. All you guys have just been terrific!