I didn't realise the percentage of people with colour blindness was so high. That's a lot of people to ignore when you're designing for a mass audience (and probably one or two even when you're designing visual material for a class).
Businesses cause problems for colour blind people when they use colour alone to convey information. This mistake is most frequently made in relation to the design of maps, diagrams, graphs, charts and other types of infographic.
These problems seem to be made worse when the coloured items are small. That is, some colour blind people report that they can only distinguish certain colours if they have sufficient “mass” (for example, they might perceive a thick coloured line as being red, but a thin version of the same line as being black). This is a possibility rather than a confirmed trait, but it’s worth bearing in mind.
Interactive digital systems providing ticketing are common culprits in using colour alone to convey information. These typically provide seating plans to help their customers choose the best available seats for the event or service that they are booking.