Accessibility: Visual guidelines - Colour, text and layout
Designing for colour blind users
Colour blindness means that someone has trouble telling certain colours apart. It is a condition more common in men than in women and there are particular combinations of colours that cause more problems than others.
Red/green colour blindness is the most common form. This means that if someone with this condition looks at areas of green and red colours of the same intensity side by side then they will have difficulty telling them apart. Avoid having red and green next to each other. Be careful when designing computer games, for example where a character wearing red might move across a green background.
It is a good idea not to rely solely on colour as a means of distinguishing objects on the screen. Where possible, combine colour with some other effect such as using a different shape, border or texture.
Tip: A good tool to use for quick assessment of web pages or images for obvious problems for colour blind users is one called Vischeck. Graphic designers/artists should download the Vischeck palettes and apply them to project artwork/page designs/mock-ups to identify any potential problem colour combinations in chosen colour schemes.
Web-safe palette
GIF files, text and background colours on a web page should use colours from the web-safe palette.
If your project will use graphics stored as GIF files then use the web-safe palette for such images. This palette, also known as the Netscape, browser-safe or web palette, is a reduced set of colours to choose from, 216 in total, which ensures that graphics will render properly on a range of display devices. For example, artwork designed on a Mac will look similar (note: not identical) on a Windows machine and vice versa. If the colour you want to use is not in this palette then either 'snap' to the nearest equivalent or use dithering techniques, or switch to using JPEG format.
If you are specifying background and foreground colours for text to use in a stylesheet then these too should be colours from the web palette. Graphic artists should ensure that any editing tools they use have options for choosing colours from the web palette. Most recent versions of tools such as Photoshop/Freehand/Fireworks will have this functionality built in.