It's a bit more complicated, but you can do it. You need to use something like .tmp to store the color, have a check like:
if sim.partProperty(0, "tmp") == 0 then
    sim.partProperty(0, "tmp", math.random(1,5))
end

This sets .tmp to a random value from 1 to 5 on creation.

Then you need to create a graphics function, there is some information on that here: https://powdertoy.co.uk/Wiki/W/Lua_API:Elements.html#Properties

return 0 as the first value (which means, don't cache result), then change some of the other values depending on .tmp