Screens on computers and mobile devices are getting better in 12 month cycles. I design presentations on 2 giant 27" displays that show a lot of detail and reveal the most subtle color shade differences.

Then you put your presentation on a conference room overhead projector that has been sitting there since 2002... The screen resolution is so small that it takes you 5 minutes to find your application windows, and when you finally get to show the slides in presentation mode you will notice that white and light grey is the same color, and that almost black grey is actually bright grey, and that orange means pink.

Test run your designs for the crappy conference room projector.