Painting - adding a stripe when painting interior wall

My son has requested a 3-tone wall, 2 shades of green - one on bottom, one of top, separated by a white or very light green stripe. Any suggestions for method or material ( what kind of tape would protect best against bleeding). Thanks for any help.

Blue painter's tape works best, made by 3M or Scotch. Make sure its layed down real tight at the edges. For a really crisp line, try bridging. Using light-weight non-shrinking spackle, (Red Devil makes the best for this), take your finger and smear it in real good on the edge of the tape. Rub it in until you can't see it anymore. Let it dry, and paint the stripes. Pull up tape immediately after painting, or after final coat is applied.

Thanks! I'll buy it tonight. Just one more thing. Do you have suggestions on how to keep the tape straight? His room is 12 x 12, and I can't think of a method for applying the tape in a straight line. Big pieces? Little pieces?

Either use a 4' level, and make small pencil dots every couple of feet or so, or use a laser level. Try to keep the pieces as long as you can manage, so there will be minimal points of start-stop or overlap with the tape.