Microsoft Adaptive Cards do not allow random background colors.
The suggested workaround was to use images. Instead of creating an image for each color, this small Javascripts will generate a data uri for a single-colored GIF image.
Please see this page for background information about the workaround.
Your preferred color:
"backgroundImage": {
"url": "data:image/gif;base64,R0lGODdhCAABAIAAP///wAAAACwAAAAACAABAAACA4RvBQA7",
"fillMode": "Repeat"
}
Created with Coloris by Momo Bassit.