Using your brain to control your heart to control your penises
Sherlock introduces the stag night as an, ‘embarrassing’, story. What exactly about the stag night was embarrassing? Not getting excessively drunk: that’s par for the course. So, what is Sherlock hinting at, here? Something embarrassing happened at the stag night.
The story begins with his visit to Molly. He shows her a file with a picture of Vitruvian man with John’s head pasted onto it: this explains his anxiety, he’s going drinking with the perfect man, he doesn’t want to lose control of his penis, in the closet. From this we already see that Sherlock is apprehensive about losing control with John because of drinking.
Once they begin to drink John sees, and is annoyed by, Sherlock’s beakers of beer plus his phone. It is obvious to him that Sherlock is going to attempt to control their drinking in with a bit of science,
So… the phone symbolizes Sherlock’s heart via its association to Irene’s phone. The beakers, long, cylindrical and filled with yellow liquid represent their penises. The app, the graphs, Sherlock’s measurements are symbolize logic, his brain, his rational mind, Sherlock trying to control the events of the night with his brain.
So, basically we have Sherlock using his brain to control his heart to control their penises,
(The screen cap above, I think makes an allusion to John’s heart, as well. The graphic of Sherlock’s stop watch disappears into John’s back. The red rectangle on his left side. Logic will control their hearts and their hearts will keep their penises in check.)
At the climax of Sherlock’s, 'system’, he is very well lit in blue light, a blue glow all over him,
As soon as John rebels against Sherlock’s way of drinking we get the scene where John lets loose and starts drinking away from Sherlock’s control. Here we see the clouds of the bisexual flag very clearly in the background: blue for opposite sex attraction, pink for same sex attraction and purple for its synthesis,
Once we have this explicit key we can look back at the colouration of the previous scenes and see the story that the colours tell.
The various bars that they go to all have varying degrees of these three colours. To me, this shows that the whole night, they’ve been oscillating between getting close and getting far away. Sherlock tries to keep things, 'blue’, AKA straight between them, this is especially clear right when he’s asking about John’s, 'maximum volume discharge’, ironically an incredibly sexual thing to ask. This is right before John’s rejection of this repression. This is Sherlock trying his hardest to be straight in this context, hence he is bathed in blue. John, by contrast, is only faintly framed in blue, his face and body are not in blue, at all.
At the second bar we see, there is a very large phallic statue between them. It casts two penis-shaped shadows: one tall and faint and one shorter and darker. The latter also appears to be coming from John’s direction and pointing at Sherlock. To me, this could represent John’s arousal vs Sherlock’s blue tinged, arousal. Here John has pink light hitting his face and the shadow directly behind him is also pink. Though the left side of JOhn’s face is blue (the side of the heart), the middle of his face is still pink and he has a symbolic boner behind him,
At this club Sherlock shows his incredible and supposedly non-existent drinking prowess in a gesture I can only see as symbolic. He polishes off that whole penis of beer and then does a mouth wipe surrounded by purple,