Are you using a template? I don’t think you can add a color with the standard create-note option, but you can make a template in Agenda with a background color, and create new notes from that template.
Eg.
agenda://x-callback-url/create-note?title=My%20Note&text=content&template-name=Your%20Template%20Name
There’s also a template-input parameter you can pass alongside it to fill in template variables. X-callback-url Support and Reference