I've added some helper functions for setting your post processing properties in javascript to my itch.io page. Feel free to include them in your API upgrade.
I was unable to get any of the following post processing properties to work:
"Brightness" float (0-1)
"Contrast" float (0-1)
"Saturation" float (0-1)
"Vibrance" float (0-1)
"Tint" color as vector3d(R,G,B)
"Hue" float (0-1)
"Vig" float (0-1)
"VigRadius" float (0-1)
"VigColor" color as vector3d(R,G,B)