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

lib/display/failure.ex

@@ -1,10 +1,12 @@
 defmodule Display.Failure do
+  alias Display.Paint
+
   @no_value :ex_unit_no_meaningful_value
 
   def format_failure(%{error: %ExUnit.AssertionError{expr: @no_value, message: message}, file: file, line: line}) do
     """
-    #{Colours.cyan("Assertion failed in #{file}:#{line}")}
-    #{Colours.red(message)}
+    #{Paint.cyan("Assertion failed in #{file}:#{line}")}
+    #{Paint.red(message)}
     """
   end
   def format_failure(%{error: %ExUnit.AssertionError{expr: expr}, file: file, line: line}) do
@@ -12,21 +14,21 @@ defmodule Display.Failure do
   end
   def format_failure(%{error: error, file: file, line: line}) do
     """
-    #{Colours.cyan("Error in #{file}:#{line}")}
+    #{Paint.cyan("Error in #{file}:#{line}")}
     #{format_error(error)}
     """
   end
 
   defp format_assertion_error(error, file, line) do
     """
-    #{Colours.cyan("Assertion failed in #{file}:#{line}")}
-    #{Colours.red(Macro.to_string(error))}
+    #{Paint.cyan("Assertion failed in #{file}:#{line}")}
+    #{Paint.red(Macro.to_string(error))}
     """
   end
 
   defp format_error(error) do
     trace = System.stacktrace |> Enum.take(2)
-    Colours.red(Exception.format(:error, error, trace))
+    Paint.red(Exception.format(:error, error, trace))
   end
 
   def show_compile_error(error) do

lib/display/intro.ex

@@ -1,4 +1,6 @@
 defmodule Display.Intro do
+  alias Display.Colours
+
   def intro(module, modules) do
     if not module in modules do
       show_intro(module.intro)

lib/display/notification.ex

@@ -1,6 +1,8 @@
 defmodule Display.Notifications do
+  alias Display.Paint
+
   def congratulate do
-    Colours.green("\nYou have learned much. You must find your own path now.")
+    Paint.green("\nYou have learned much. You must find your own path now.")
   end
 
   def invalid_koan(koan, modules) do
@@ -13,7 +15,7 @@ defmodule Display.Notifications do
     |> Enum.map(&Atom.to_string/1)
     |> Enum.map(&name/1)
     |> Enum.join(", ")
-    |> Colours.red
+    |> Paint.red
   end
 
   defp name("Elixir." <> module), do: module

lib/display/progress_bar.ex

@@ -1,4 +1,6 @@
 defmodule Display.ProgressBar do
+  alias Display.Colours
+
   @progress_bar_length 30
 
   def progress_bar(%{current: current, total: total}) do