For example, most links have both a color and an underline. If you use style sheets to remove the underline, the color remains the only cue that there's a link. I've seen sentences in which one different-color word with no underlining contains a link to its definition. Color-blind users would have no cue that there's a link there. One solution would be to have a small graphic of a book next to it.
Color is a particular problem with imagemaps in which clickable areas are delineated by color.
The areas could be enhanced by placing underlined text or a black outline in the image. The rollover effect with changing color is often used to denote an image as clickable. Displaying an arrow could enhance this. Placing links together as a menu along the top or down the left side also helps identify them as links. Not only can most color-blind people see black and white accurately, but they see all shades of yellow and blue.