A fully black beard is a sign of distress which would be more urgent, but there are gradients of darkness that could range in a grey-scale implying moody or stressed, etc.
How old is he? Based on you stating a yellow beard as normal, I take it we're talking about an adult.
How black/dark is his beard getting? Does it happen routinely or seem continuous?
If you could run us through what bulbs you are using as well we may be able to pinpoint what is causing it. What do you mean by throwing punches at the light?
He keeps nodding all the time! It's been driving me crazy, he can't walk anymore without doing the nodding thing with his head. Is this due to him being in heat/ ready for a female? Or does he do it because he sees himself in the reflection?
It is breeding season so it's quite possibly that as the head bobbing/black bearding are also a sign of hormones or dominance behavior. Has he humped anything yet?
Not that i've noticed, but he has been digging alot. His beard isn't black right now, so I guess it's not that dangerous? Aince it's more of a greyish tone and he can change it.