Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In Canto One of his poem John Shade (the poet in VN’s novel Pale Fire, 1962) says that, when he was a child, all colors made him happy, even gray:
All colors made me happy: even gray.
My eyes were such that literally they
Took photographs. Whenever I'd permit,
Or, with a silent shiver, order it,
Whatever in my field of vision dwelt -
An indoor scene, hickory leaves, the svelte
Stilettos of a frozen stillicide -
Was printed on my eyelids' nether side