>If grey hair is energetically no more or less favourable than coloured hair then it would be difficult for it to spread as an honest signal of unfitness because it could just as easily be faked.

You're right. It's not an honest signal, because it can be faked.

However, while it's advantageous for individuals to fake, it's disadvantageous for the populations that contain individuals that fake.....so there's pressure to fake and pressure not to fake. Overall I would guess the pressure not to fake would outweigh the individual pressure to fake but it is just a guess...

>It seems more likely that grey hair is a signal for something else - age and hence some kind of experience/authority, or a side-effect of something else."

I agree. I showed a possible reason for gray hair, I am sure there are others. ANd in reality the total advantages for grey hair are going to be a SUM of the advantages and disadvantages. Whether that sum is a total positive or negative, who knows; the sign (+ or -) will probably differ depending on whether we're talking individual advantages or population advantages.

Which leads back to my original point; that some of the adaptations dismissed as "quirks" probably aren't.