In a word, I love this game. Really. It's smart and well done.
This game is well polished. Grayscale could indeed make it beautiful, but I really don't think it's necessary.
You could add something funny: when the time is nearing zero, the screen (or a part of it) could starts flashing (to make the user panic).
And BTW, without Dragon Ball Z I would never would have guessed the awnser to the first puzzle.
[EDIT:] And BTW, without Dragon Ball Z I would never would have guessed the awnser to the first puzzle when I launch VTI and your program for the first time. Maybe you could add some kind of "randomize()" ?
Thanks for the comments jfg and Stoopid Guy. I don't think I really want to do gray scale at this point. I may find some type of design to draw in the background in light gray and draw everything else in black. jfg, would you mind playing the game a few more times and seeing if it gives you the same puzzle? I do use randomize but maybe you have found a bug.
Nevermind what I said.
I don't think its a bug because each time I rerun your program, I restarted VTI at the same time, so it's kind of normal.
Also, some kind of highscore system is really important. You could show for instance the percent of the words that have been discovered.