What do these gray note heads mean?

thanks! learning Musescore now and your answer help me too!!!

See the description of the "visible" property at https://musescore.org/en/handbook/inspector-and-object-properties#categ….

There are two possible reasons a notehead would show up 'grey' in a score.

1. A note may appear coloured a rather vague greyish-green, as in the first example shown below. Depending on the screen's colour settings, a new user might mistake this colour for 'grey'. (The colour difference shows up much more strongly on this webpage than it does in MuseScore itself.) This grey-green colour is the result of entering a note which is outside the 'amateur' range defined for that instrument in the staff properties dialogue. It is a warning the program gives users that if they are writing for amateur players, they might want to reconsider asking them to play that note as it could be too difficult.

This grey-green colouring of the note ONLY shows up in the score while you are working on it in MuseScore itself. It does not affect playback, and it will print or export to PDF as a normal black note (unless you deliberately change the notehead's colour using the F8 inspector).

2. If the notehead is well and truly grey, it means the user has (deliberately or by accident) set that notehead to be 'invisible.' That is the case in the score posted by the OP at the beginning of this thread, and in the second example shown above. A selected note can be set as invisible either by unchecking the □ Visible box in the F8 inspector, or by toggling visibility off by typing the letter 'v' (type 'v' a second time to toggle visibility back on).

Note that to see the grey notehead at all, you must have checked Show Invisible in the View drop-down from the main menu. Also note that noteheads made invisible will not print either on paper or in a PDF export score. Finally, if you DO want a note to be completely invisible, you must select both the notehead and the stem before setting them as invisible. The program treats noteheads and stems as separate elements.