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ArcGIS REST API: ColorScheme
Needed to look up ArcGIS's JSON color settings as served by the REST API. So it is listed on their Common Data Types -> Symbol objects and they indeed use RGBA.
To quote from ESRI's website:
Color is represented as a four-element array. The four elements represent values for red, green, blue, and alpha in that order. Values range from 0 through 255. If color is undefined for a symbol, the color value is null.
[ <red>, <green>, <blue>, <alpha> ]
[ 67, 0, 255, 40 ]
Also worth a read: https://en.wikipedia.org/wiki/RGBA_color_space