Should be pretty easy, there's a Vexflow demo with about 20 lines total.
0xfe/vexflow#674
http://jsfiddle.net/9cd3s0v2/4/

This should be an OSMDOption that accepts a background color and stroke color.

bracketSpan@func documentation:

https://music-encoding.org/guidelines/v4/elements/bracketspan.html

func(required)Describes the function of the bracketed event sequence. Allowed values are: "coloration" (Represents coloration in the mensural notation source material.), "cross-rhythm" (Marks a sequence which does not match the current meter.), "ligature" (Represents a ligature in the mensu