Interesting Sudoku Logic

I enjoy working the Sudoku puzzles in the newspaper.  Starting on Monday, the difficulty increases from 1-star to 6-stars on Saturday, with a 5-star on Sunday.  I find I can nearly always work 1- through 4-star puzzles without making any marks on the paper except filling in the numbers.  Unless I want to put in 30 minutes or more of deep thought, 5- and 6-star puzzles usually require some guesswork that I write with small numbers in the corners of the unfilled squares.

Today's 5-star puzzle, however, I managed to work without guesswork, using a logic sequence that I cannot remember using before.  Here is the partially worked puzzle, with a few key squares shaded:

For you logic fans, where should the "2" go in the upper-left corner?

Here is the interesting logic sequence.  Notice that the green squares must contain "2" and "4".  In the upper left, the "2" can potentially go in either the blue or red boxes, and a "4" must go in one of the blue boxes.  But I happened to notice that if the red were not a "2", then both the blue and the green boxes would contain "2" and "4".  But this can't be, since then the solution would not be unique-- if the blue had the "2" on top of the "4" then the green would have the "4" on top of the "2".  But an equally valid solution would have the blue "2" and "4" switched, with the green also switched.  Thus there would be two valid solutions, which is not allowed.  The conclusion: the red box MUST contain a "2". Putting a "2" in the red box made the rest of the puzzle easy to solve.

Here is the full solution.