Unlike Neymar, Costa has only just broken onto the big stage and doesn't have a bad rep built up like Neymar did with his publicity, diving, and all that. To me, this proves it's the trick itself that leads to the reaction. I'd say the connotation of a rainbow flick is the same for Costa as it is for Neymar. But Costa's rainbow looked like the absolute best thing he could do, Brandt wasnt even close to defending that.

its very unlikely to go through there while holding the ball to the ground, because there is absolutely no space. Even with an impressive ground-footwork á la Dinho, R9, Messi etc. If you look at Brandt there, he does a step to the left, to close the space between him and the sideline. The situations that Costa has been in have always somehow justified to do skillmoves because they helped. And Costa has so far performed an incredible skillset, but none of them has ever included stare-offs, standing step overs, leg shaking, Rabona-stops/passes or other sort of footwork with the only purpose to humiliate someone. I do usually see rainbowflicks as amazing and underused skills, not too hard to pull off actually, but absolutely unexpected and often efective if correctly pulled off. He goes straight forward and the rainbow seems to be a quite usefull rather than unnecessary skillmove to go there. But Costa here in that situation, he is kinda surrounded by defending players on the wing.

Costa rainbow flick