Successful use of a web page may depend on colour differences and contrast levels as much as the shape of symbols where colour is used to distinguish items. Size of fonts, their style and the ability to read text against certain background colours is also important and designing in black and white before adding colour can help. Problems may arise where there is no distinction between links and visited links except for the colours. Further advice from WebAim Designing for Color-blindness.
These tests contribute towards accessibility scores for this disability.