Ever since the Super Saiyan transformation was introduced, we have seen Goku’s hair change from black to gold, to red, and lastly, to blue. And now with Ultra Instinct, Goku’s hair changed into black to black tinted with a silver aura. This is the most unusual hair color in every Goku’s transformation so far.
It is not only unique by its color, it is also unique because it is not Goku’s usual Super Saiyan spiky hair nor his base form hair. Also, it is not classified as Super Saiyan because not only Saiyans can achieve this form. That does not explain the reason why Goku’s hair turned gray in his mastered Ultra Instinct form.
The real reason might be because the Ultra Instinct is not really a form made for humans. It might be a feat to achieve for humans and GoDs alike, but it is very common among angels. Ultra Instinct might be the basic requirement for an Angel to be qualified to work with a God. For the GoDs, it would be their greatest feat and will open up the doors for being as powerful as their angels.
Goku’s hair turned into gray because this state does actually the hair of anyone who achieves it into gray. That is why Whis and the other angels have silver hair. They have it permanently because they are in Ultra Instinct state permanently. Goku’s silver hair only appears when he transformed into that state.
It is kinda obvious why Beerus and the other Gods of Destruction don’t have a silver hair. In addition to the fact that most of them are cats and reptiles, they haven’t mastered the Ultra Instinct yet. We haven’t seen how two Ultra Instinct fighters can actually have a fight with a winner, but with Jiren and Goku’s fight, we might actually witness it.