I'm playing this great game on my mac. The problem is that I am colourblind, so I can't always see where I am shooting (especcially on a dark background). Can I change the colour of the +-sign (aim-sign) for example into yellow or white?

i only know how to change almost everything into yellow

Yes, you can change the color of the crosshair (+ sign) in 'Player Setup' are on the multiplayer window. To change it to white drag all the color sliders (down at the bottom right) to right, to have yellow, leave all sliders to right but drag the blue color slider (the bottom most one) all the way to the left.
You can experiment and see which ones would fit you best though.

Hope this helps, cheers.

Thanks! That worked for me!