White text + colored background = invisible

I recently had the opportunity to help seven undergraduate students prepare for a presentation at a regional conference. These people are some of our best students. Four out of the seven students had slides that looked like this:

White text on a colored background becomes nearly invisible in our classrooms. It's washed out. Turning off all of the overhead lights doesn't help very much. There is too much ambient light coming in from the windows and an emergency overhead light that cannot be turned off. 

The slides that had black text on a light background had much better visibility. It's a better sensory combination for most classrooms. 

Slide designs and color combinations that look good on your office computer can completely fail during a presentation. The only way to be sure is to test the appearance in the classroom.