This purple pattern flickers on and off in front of Sherlock’s wound, John’s face and Sherlock’s face during the ambulance drive.
It is hidden in the middle of the flickering of the ambulance blinkers, but it differs from the light of the blinkers radically. The light of the blinkers is blue and white, it doesn’t flicker very bright inside the ambulance and it creates shadows. The purple light doesn’t create shadows and the pattern of it is the same in every shot, even in the mind palace. This means it is placed there afterwards, and someone placed it there on purpose.
Purple lighting is commonly used throughout the series. We see it in many scenes with great importance for the relationship between John and Sherlock: in ASiP during the dinner at Angelo’s, when Sherlock rejects John, and in the same episode in the Sherlock’s and cabbie’s two pills -scene behind a window on the background, before John saves Sherlock’s life. In TEH there is purple as well on the background when Sherlock rushes out of 221B to save John’s life, and in TSoT when Sherlock leaves the wedding after John has rejected him and laughed at those rumors. All these scenes make something shift in their relationship. They are scenes of feelings. If there is a theme color for the relationship and feelings between John and Sherlock, it is definitely purple.
Here in the ambulance scene we have a rapidly flickering purple pattern. The color symbol of their feelings highlights this:
The theme color of their relationship flickering and fading, flashing on their faces and on the wound like an alarm. I don’t know what the pattern is or if it matters, but the color is there for purpose: it tells us this scene is important. This is about their relationship and their feelings. They are flickering, they are panicking.
And because Sherlock has already retreated into his mind palace and is barely conscious, this scene is especially from John’s POV. It means the panicked flickering represents especially John’s feelings. He is concerned, afraid, terrified. He sees their relationship flicker like it’s struggling to exist.
Wound, John and Sherlock. Then into the mind palace: the pattern fades away, flickering dies.
We didn’t get to see weeping John or a bedside scene. The cinematography tells us what he felt.