When your born you see no colour, but when you first touch your soulmate you see every colour under the sun.
If your soulmate dies your world turns black and white once again.
Sam saw colour twice but lost it.
John saw it once and lost it.
Dean has never seen colour, he's lived in black and white since the day he was born.
It was the same for Castiel.
Until he raised the righteous man from Perdition.
Dean never understood why he could see colour when he came back from Hell, but never questioned it.
Until Castiel died for the first time and his world lost every drop of colour.