Automatic coloring clef/key signature changes

[Jérôme Plût]

Is there a way to change the color of all clef changes in a score (and
only these, not the clef that is at the beginning of each staff line)?
I cannot manage to change even one of these (I tried \once \override
Staff.Clef.color, to no avail), and what I really want is to change
all of these by a one-time incantation at the beginning of the score.

Also, same question for KeySignature.

Thanks,

-- Jérôme