To improve testing, add a layer of indirection around colours.

lib/display/colours.ex

@@ -0,0 +1,36 @@
+defmodule Display.Paint do
+  def red(str), do: painter().red(str)
+
+  def cyan(str), do: painter().cyan(str)
+
+  def green(str), do: painter().green(str)
+
+  defp painter do
+    case Mix.env do
+      :test -> Display.Uncoloured
+      _ -> Display.Colours
+    end
+  end
+end
+
+defmodule Display.Colours do
+  alias IO.ANSI
+
+  def red(str), do: colourize(ANSI.red, str)
+
+  def cyan(str), do: colourize(ANSI.cyan, str)
+
+  def green(str), do: colourize(ANSI.green, str)
+
+  defp colourize(color, message) do
+    Enum.join([color, message, ANSI.reset], "")
+  end
+end
+
+defmodule Display.Uncoloured do
+  def red(str), do: str
+
+  def cyan(str), do: str
+
+  def green(str), do: str
+end