Just by adding the gem redgreen, the 'rake test' always finish by the line :
0 tests, 0 assertions, 0 failures, 0 errors
I've created a example project on github: https://github.com/joel1di1/test_colored_tests
In your Gemfile, change the group from being both :development and :test to just :test.
This way the redgreen gem is required in when you are running tests.