Search for color blindness simulation tools on the net. You will find for example
- Sim Daltonism (Mac OS X): http://michelf.com/projects/sim-daltonism/
- Colorblind Web Page Filter (Web): http://colorfilter.wickline.org/
- 15 Tools If You Are (Not) Colorblind: http://www.colblindor.com/2008/12/23/15-tools-color-blindness/