<p>I believe I already know the answer, but I am not 100% sure, so just a quick question: What does Red/Green Testing actually mean?</p>

<p>I understand it as "Write your tests first, so that they all fail (= all red), then write your code and watch how each test turns green, and when all are green, you're fine".</p>

<p>I heard this in <a href="http://sessions.visitmix.com/?selectedSearch=T22" rel="noreferrer">Scott's MVC Talk at Mix</a>, so I do not know if this is an "official" term or if he just made it up. (Edit: Scott actually also explains it starting at 55:00 Minutes, and he made a good remark why he beleives in it)</p>

It does refer to TDD or Test Driven Development, but it would apply to each test.  Write the test first, then write the code to pass the test.  It would be wrong to write ALL the tests first.  TDD is an incremental development approach.
The basic idea is no code is written before there is failing test (RED).  When you have a failing test, then you write the code to pass the test (GREEN).  Now you are ready to write the next test -- i.e., no new tests until all are green. Or refactor, as @Brian points out.