Is it showing off? Or is it merely a demonstration? Jingyi may never really know, Lan Wangji's face as impassive as ever. The answer can be maybe argued for, in that Wangji turns on his hand slightly to better watch Jingyi show off his tail.
"Interesting. Benefits and draw backs in all things." he muses, admiring the colour.
"Is the color pattern linked to the personality or particular skill?"

He trusts Jingyi to tell him he's being intrusive.