Providing pages that have graphics but that work when graphics are not available takes some practice. Alt text should always be written to replace an image, rather than to describe it, if at all possible. If it is not possible, then the image should not be critical to the message. If an image has null alt text and is also a link, the link needs to be duplicated in text on the same page. Image maps are useful for many cases, but it's important to provide text alternatives in addition to the images.
Color is a complex and multi-faceted problem in itself, but there are a few simple guidelines.
It is extremely important to begin all design in black and white, and to use color for emphasis and enhancement. Never use color as the sole distinguishing characteristic of anything. Colorblindness involves a partial or complete insensitivity to certain color ranges. Computers can exacerbate this problem because they rely on only three color wavelengths to simulate the thousands of wavelengths in the visible spectrum. Remember that any insensitivity to a particular color means that for a colorblind person, that color will disappear when intensity drops below the visibility threshold.