VS Code has really nice extension 'indent-rainbow'(~800k installations)
"This extension colorizes the indentation in front of your text alternating four different colors on each step". Really helps to improve readability.
It would be nice if Scintilla can support such function
That seems similar to this example which used Scintilla's indicator feature.

Sorry, I'm a little bit confused.
You mean it's already possible?
Scintilla supports showing areas of the text differently using 'indicators' which can be used by an application or plugin author to provide a particular feature like rainbow indentation.
That seems similar to this example which used Scintilla's indicator feature.
