Add test to guard against unused colours

test/test_language.rb

@@ -494,4 +494,11 @@ class TestLanguage < Minitest::Test
     message << missing.sort.join("\n")
     assert missing.empty?, message
   end
+
+  def test_no_unused_colours
+    Language.all.each do |language|
+      next unless language.type == :data || language.type == :prose
+      assert !language.color, "Unused colour assigned to #{language.name}"
+    end
+  end
 end