I managed to find my matmatic sources in an old dropbox folder. The rgb method was introduced in matmatic 1.7. There were several changes to make it easier to create colors either using integers or floating point numbers and not have to keep changing between Color and IColor. As well there were changes to make it optimize smarter, where sometimes a node network would be generated, and sometimes simple constants.

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Matmatic 1.7.0

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Added AmbientOcclusion parameter EvaluateInIDL.

Added PoserSurface parameter Gradient_Mode.

Added function rgb - constructs a color from three components. This is a smart function to construct colors statically (at compile time) or as nodes during the render. If any component is a node, a UserDefined node is created. If any component is a float, then Color(r, g, b) is returned. Otherwise, it returns IColor(r/255.0, g/255.0, b/255.0). Thus it is possible to express using either 8-bit (0 to 255) integer style or floating style (0.0 to 1.0).

Added function max(*args) - returns max of its arguments using compile-time math or numeric nodes or color nodes

Added function floor(x) - can take numbers (returns math.floor(x)) or nodes (returns Floor(x)) - thus it does the right thing.

Added function hsv(hue, saturation, value) - returns an rgb color (interprets int value as 0 to 255, float value as 0.0 to 1.0). This function will produce an exact color of all three inputs are known at compile time - otherwise it will create a node network that does the job at render time.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)