One thing you can do, if you have steady hands, is buy yourself a good, broad brush (like a 3 inch Purdy) and make nice long, smooth strokes along the wall with your green paint. With a little practice, you'll be surprised how straight you can paint a line. Otherwise, when you run your masking tape, run it in the corner exactly where you want your line to be. Then drag a stiff putty knife along that edge, pressing firmly. This won't guarantee that no paint bleeds under the tape, but very little will get through. Good Luck!