Fade to Gray

Often used as a "trip down memory lane" effect, this occurs in film or TV shows (usually referencing a film) when wishing to confer an obvious stylistic nod to the black-and-white era of moviemaking upon the audience. A simple trick, the saturation (colour) of the effect is slowly drained until the picture is completely monochromatic or grey, just like all those Film Noir pictures of the 1940s and 1950s. The effect can also be used more subtly, with only a slight change of hues and saturation to simulate the effect of a dream (fuzzy edges), fantasy (usually with a cloud) or flashback sequences as required. Also done in reverse.

A Sub Trope of Deliberately Monochrome.

Film

 * Clerks II used this at the end of the movie as a nod to the black and white origins of the first.

Western Animation

 * In The Simpsons episode $pringfield (Or How I Learned to Stop Worrying), Jasper and Abe are visiting a movie parlour back in their day as young men, watching a short film about the successes of Springfield. This is shown in black and white before fading to colour, revealing them as hypocritical old men. The current events movie they watch also contains a few Department of Redundancy Department jokes.
 * The Simpsons also did this in an early issue of Simpsons Comics, wherein a tale Grandpa was telling of his past, the pages would randomly turn black and white. Bart complains about this.