Program behaves different on different machines

Hey!

I hope that someone can help me with this problem. I want to be able to share my program so others can use it, but for those I have send it to, the program is acting bananas. I have a link to an early version of the program. Some things have changed since then, but you should be able to understand what it does:
http://www.youtube.com/watch?v=AbebPk5VlPw

But when others try the program, the colours become almost epileptic. Not only that, but the brush don’t travel across the whole canvas.
I guess that the first problem has to do with a “tick” I increase every frame. I tried to lock the program to use a constant fps, but the problem still is there.
And the other problem may have to do with ofGetScreenHeight() and ofGetScreenWidth() that I use. I had problem using ofGetHeight() and ofGetWidth(), since they didn’t give me the right values.

I will share my code, and hopes that someone can help me to find a solution.
[attachment=0:3uj2nle5]src.rar[/attachment:3uj2nle5]

src.rar