https://www.campaignmonitor.com/css/
https://www.litmus.com/blog/background-colors-html-email/
Stick with the CSS properties, and use 3 or 6-digit HEX codes or RGB values on table & td. Those worked across all email clients.
Introducing opacity with the alpha for the RGBA value wasn’t supported in Outlook, Web.de, or GMX.de, but had decent support otherwise.