Credo.CLI.Output.foreground_color

You're seeing just the function foreground_color, go back to Credo.CLI.Output module for more information.
Link to this function

foreground_color(background_color)

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Returns a suitable foreground color for a given background_color.

iex> Credo.CLI.Output.foreground_color(:yellow)
:black

iex> Credo.CLI.Output.foreground_color(:blue)
:white