Fades to Black and White
From Harvard Medical School (Paywalled Link):
To someone who is depressed, the world can seem flat or dull. This was long thought to be purely psychological. It turns out, though, that depression may affect how the eyes function—altering visual perception in a way that actually makes the world look gray, reports the November 2010 issue of the Harvard Mental Health Letter.
I remember trying to describe this to people, and they thought I was exaggerating for dramatic effect.