How do the card cover background colors get chosen?

[Caryn Martinez]

When I add an image to a card, it becomes a cover. When the card is closed, you just see the image. When the card is open, there is space to the right and left of the cover image that Trello randomly colors. Sometimes it's nice, like black, and other times it's a strange color, like an ugly green.

How does Trello pick that color, and how can I change it? 

[Iain Dooley]

@Caryn Martinez it's just a calculated average of the colours in the image, you can't edit it "officially" but you might be able to find a chrome extension that allows you to change it.