Sherlock doesn't quite understand why he's jealous, but he knows he is. It bugs him.
Jealous
"Keep in touch, of course," purred Mycroft.
"Yeah, definitely!" beamed John.
Sherlock glared at his coffee mug (his favorite— a massive, sea green one).
Somehow, his brother and John had started talking and had managed to pass an hour getting acquainted. John had treated Mycroft to coffee, making him the same thing he'd made Sherlock, the mochaccino (Sherlock did not like that). Now they were chatting like old pals. He found he rather disliked the way John leaned on his hand and smiled when talking to Mycroft.
He added Seduced John to his list of reasons for hating his brother.
Prompt was #48: sea green
Maybe "seduced" wasn't the right word (though Sherlock would take it to such an extreme, wouldn't he?). Perhaps "enchanted"?