CSSEx.RGBA.new

You're seeing just the function new, go back to CSSEx.RGBA module for more information.
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new(r, g, b, a, base \\ 10)

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Generates a %CSSEx.RGBA{} wrapped in an :ok tuple, from the values of r, g, b, and alpha. All values are treated as decimal by default but another base can be provided as an optional argument.