On the Web and when using computers, many people with low vision use specialized monitors or software that increases the size of text or images large enough for the individual to see. Web sites that use absolute font sizes make it difficult for the low vision user to make this adjustment using his or her computer. Additionally, some low vision users have difficulty making out certain font styles. Italic text, for example, may be difficult for a low vision user to read without assistive software. Cones pick up colour.
Rods pick up brightness.
There are blue cones, red cones and green cones. They pick up different wavelengths of light. Colour blind people have less numbers of particular cones than normal, so they get colours confused. Some people are more colour blind than others. Web accessibility issues for individuals who are color blind often involve color combinations that are not properly coordinated or do not provide high contrast.