Everything works great on screen only doesn't it Welcome to the slightly disappointing world of printers...

1. Very few printers will actually print totally opaque, especially on transparent sheets. Especially if it's an inkjet, but even lasers won't do it completely either.

2. You will have to have at least a six or eight color [inkjet] printer to print white, and believe me, it is NOT worth it!!! White ink is known to cause some problems and is very hard to keep flowing smooth. So in short, no, you can't print white. Unless, of course, you have a huge thermal printer with spot color capability. And that's correct, anything "white" on-screen will print NOTHING in it's place. (CMYK value of 0-0-0-50 would of course mean you have a 50% [50 black, 50 white, or in other words, nothing] black fill, so half of that object will be transparent when printed!!)