It shows how Sherlock and Molly are perfect complements of each other.
With respect to color, there is a yin and yang quality to the stark contrast between the color of their coats - black for him and white for her.
The lighting proves the same.
In a similar way you can see that despite her light colored ensemble, Molly’s face is darkened by shadows to the point where you can’t even see her facial features. Looking to Sherlock, despite the darkness of him, his hair and heavy coat, his face is bright, light and very visible.
It is as if within their respective wash of colors, white for Molly and black for Sherlock, there are dollops of the other - the shadow on her face and the light on his.
It is as if this kiss was to show that they are the embodiment of the perfect balance.
Molly is good, awkwardly so, but she can also harbor some darkness. She is not simply the bumbling, shy, lovestruck girl. And while Sherlock is aloof and dark, he too can love deeply and feel empathy - and this kiss and scene (as well as the other one later on in the episode) with Molly is proof of it.
The Empty Eye, or, The Vision Fetish, or, Trains and Projection
I wrote a couple of posts ago that Sherlock is all about seeing–not just observing, but looking. Those blokes must have heard me, because this episode was a feast for the eyes. Vision porn galore. I seriously didn’t intend to be up this late writing meta already but this episode is just full of eye candy goodness. [I’m going to replace these caps with gifs when people make them. Or if I figure out how.]
I really think there is a significant difference in the reunions Sherlock has with John and Lestrade. John acts out aggressively You’ve wrecked my whole world. Greg isn’t mad at all. He hugs him as tight as humanly possible. My whole world is repaired.
I dunno, something about that inherent forgiveness makes me feel pretty confident in my ship.