Why don't you rather provide a service which generates more complex avatars such as those from Github[1] or Slack.

Your API consumer sends some unique string (user-id, hashed user-id or hashed username or whatever they like) and you send back an unique avatar for that string. Moreover, you as the API provider offer different themes (e.g. tablecoth, space invaders, pixel-faces, etc.) which the consumer can again slightly customize (e.g. color, size, etc.) to have a unique style over all its avatars.

You current solution is elegant but at the same time just too easy to build oneself.

[1] There was a code golf competition on SE, maybe useful in this context: https://codegolf.stackexchange.com/questions/82290/generate-...