Under My Skin

For this prompt on the Kink Meme:

In a universe where people's emotions and emotional well being are reflected by a person's skin colour or tone, Sherlock just has one small ring that he keeps well hidden.

It's a solid black for a long time, but the more time he spends with John, the more the black disappears and the more intuned with his emotions he gets, and then eventually, the ring starts growing and spreading.

Bonus:-

The lack of "mood skin" is rare but not completely unheard of

Mycroft's "mood skin" is faint and more often than not, doesn't reflect his true emotions.

Sherlock's mood "ring" is not on the back of his hand (nor anywhere explicit).