In Single Man (2009) there is a scene where George visits a bar & orders a drink, then he sess Kenny entering the Bar.
Suddenly the brown tint of the scene changes into color tint.
Why does the tint color change in this scene?
Color changes throughout the movie to reflect the human connections that George makes and the state of his mind
Here's a couple of lovely explanations I found
...it was the editorial use of colour that really left an impression on me. Throughout the film, colour creates its own subtext, with Ford and Grau creating a shifting palette: colour leaches out of George whenever he shuns the world and, inversely, bleeds in whenever he makes a human connection. The effect is like watching the sun emerge from behind a cloud.
The use of this technique is illustrated beautifully in the sequence where the lead character, George Falconer (Colin Firth), pays a compliment to a secretary who he works with. His world until that point has been dreary, but the camera closes in on the secretary’s lips and, as the parentheses of her mouth turn upward, the colour palette is saturated and the red of her lipstick is accentuated. You can literally feel the warmth of her smile.
The first component of Ford’s synesthetic display plays out visually, through an insistence on gradual changes of color. When George experiences flashbacks, the film often switches to black and white, which marks the obvious temporal separation. But at several moments taking place in the present and within the same shot, the filter of the camera shifts from darker tones to vivid primary colors. This technique initially appears to be a cinematic artifice, but it actually echoes George’s state of mind, and it brings him to life metaphorically (and by extension revives the film itself).