Maybe I'm just overthinking, maybe it's because I believe what's right in front of me.
But doesn't the old man's colour scheme kind of match Mo's?? To me it looks like Mo's looking at him and wondering about his past, his present and his future. He's finding traces of himself in tha man.
Before, Mo was all alone, not wanting to be friends with anyone. Just feared and respected, and he was okay with it. He had nothing to lose, nothing to cherish (other than his mother), he was escaping. He was but a spectator. He watched others live their rose coloured life and never wanted to be a part of it.
But now he has a place he's comfortable in. He has friends who are a menace but love him wholeheartedly, too much even and they're making him accept the fact that he's deserving of all that love. He has Tian now, someone he holds so dear. He didn't even realise when his world had an outburst of colours so livid. He's no longer a spectator.
But he remembers that things are not going to remain the same forever, Tian is leaving afterall. But after all that hardships he tackled to come where he is now, all that effort his friends made to make him love himself little by little, if he goes back to being a spectator, it'll be waste, won't it? So he'll stay and work hard waiting for Tian to come back. And even if he doesn't, he'll dance with his memories.