Supernatural has its own distinctive visual style. This includes a specific palette of colors. Beginning with the Woman in White, it's become apparent that these colors have been used as something of a visual shorthand to reinforce the impact of the storytelling.
And because I'm obsessive like that, I've collected all the information I can find in one place.
If you'd like to read some more about where the thinking about the symbolic use of color in Supernatural came from, here are some good places to start.
bowtrunckle has written about:
Yellow, Pink and White in WIaWSNB Red Demons and Yellow Psychics Part 1
Red Demons and Yellow Psychics Part 2 sadelyrate has written about:
Blue and Green in the Angel in the House And that lit a fire under me, and I went on to write about:
Pink, White and Red Green Orange Blue Eta:
What's blue and white and red all over? Blue, white and red in Season 4
Credit goes to Oxienesis for the screencap of Solomon's Key.