That one took me a while, too. Turns out it's the "tint" property! From the API:
tint
This tints the sprite a
color onscreen. This goes in RGB order; each number affects how that
particular channel is drawn. e.g. to draw the sprite in red only, set
tint to { 1, 0, 0 }.
So if you want color to be, say (150,250,100)
you can do
tint = { 150/255, 250/255, 100/255 }(since the tint goes between 0 and 1 instead of 0 and 255)