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

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