Numbers set grey scale.
Letters set colors (b=blue, r=red, y=yellow, etc).
Mouse click sets random color.
Spacebar or double-click resets canvas.
I didn't discover this, comment stolen from previous discussion (thank you Cactus2018!): https://news.ycombinator.com/item?id=24284711
Something funny is happening; as I understand it, it is a flash thing reimplemented.
Shift+w = white background
Shift+9 = dark gray background
Shift+d = black background
Also I found out the color letters aren't limited to "rgbyo", there's a color for nearly every letter.
d = black
w = white
u = umber (nice brown)
i = teal
p = cream
s = salmon
l = slightly muted yellow
Check the code at view-source:https://jacksonpollock.org/js/myScript.js for more!
"m" brings up some info about the person who made this, Miltos Manetas. Thank you Miltos!