Thank you for the CSS, that's pretty handy!
Kinda similar thing is admonitions, which we have a plugin for: it adds preset coloured blocks.
Although, your solution is much more flexible 
I had considered doing similar thing via @import url stylesheet.css but I couldn't package it in a neat "import and go" file (without creating a plugin just for that). Also in the future, I feel, it would be great to adopt those coloured blocks with template plugin to speed up the input.
In fact, I did it: here's the same note with the coloured block that allows up to 140 named colours from w3 standard (7 are already in the note). It is also a valid template for template plugin. So, once the plugin installed it could be used right away. If pasted from the plugin, it asks you to choose the colour of the block and you're done.
Colored block.jex (5 KB)