RandomColour.PredefinedtHue.define_colour

You're seeing just the function define_colour, go back to RandomColour.PredefinedtHue module for more information.
Link to this function

define_colour(hue_range, lower_bounds)

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Specs

define_colour(nil | RandomColour.hue_range(), [RandomColour.hue_range()]) ::
  map()