On company-managed projects it's possible to change a card's colour based on a query: https://support.atlassian.com/jira-software-cloud/docs/customize-cards/#Configure-card-colors

Does this satisfy the requirements of the watchers of this ticket? Thank you!

p.s. There is a feature request for team-managed projects, here: JSWCLOUD-17277 – Custom card colors in team-managed projects

Agreed the plugin works extremely well, but only for Jira Server and Data Center.  It currently does not work for Jira Cloud (technically it does but it shows an overlay on top of the board, rather than colorizing the cards directly). I spoke with the vendor and they said that the cloud API does not yet exist for this.  I entered a suggestion to ask for that: https://jira.atlassian.com/browse/JSWCLOUD-21499 if you agree please vote it up!

The original suggestion request focused on card color. The Atlassian Marketplace appears to have at least one add-on that can support coloring entire cards for Jira Cloud:

https://marketplace.atlassian.com/apps/1219454/color-cards-for-jira

My earlier comment was in reply to @Ann Heeter's comment wanting to use "stop light" coloring, related to issue DueDates. That's possible without add-ons, as noted in my comment – regardless of whether one is coloring the whole card, or just the edge. 

I believe Mykenna Cepek misunderstood the request.   The request is not "how do I change the  sliver of color on the left hand edge of the card"   Everybody knows how to do this.   The request is  "how do I apply a color to the entire background of the card"?      The Ascend Color Cards plugin enables this for Server and Data Center, but my understanding is that this capability is not yet supported on JSWCLOUD.

@Ann Heeter

This is possible in Jira Cloud using "Board > Settings > Card Colors". Use JQL like this for your red, yellow, and green:

duedate < now()

duedate < 2w

duedate < 5d

Be sure to order them as shown (red first, green last) since priority order matters for card colors (first one wins).

My specific use case on this issue is that we are already using card colors (to color the spine of the card) to identify by card type. We have a desire to automatically color cards so that there is an easy to identify color (red?!) when a color is past its due date. It would be great if we could even stop light the card background color, for instance, green for 2 weeks before due, yellow for due in 5 days, red for past due.