You can assign a folder full of images to the background of Terminal.app, and it will choose background images at random, so a folder full of dark solid colors gives you random dark backgrounds, enabling you to tell your terminal windows apart easily.
This does not seem to be supported in iTerm, if anyone else is wondering.
For example, this script[0] picks a random color preset each time a new session is created, and this one[1] picks a preset based on the hostname.
[0] https://www.iterm2.com/python-api/examples/random_color.html
[1] https://www.iterm2.com/python-api/examples/colorhost.html