A lot of devices have been created to help visually impaired computer users. But until recently, not much attention has been paid to the particular needs of one segment of that population -- colorblind people.

Colorblindness affects millions of people. It's estimated that about 8 percent of males and 0.5 percent of females in the U.S., Canada and Europe have some type of colorblindness.

In its most common form, the condition makes it difficult to distinguish between reds and greens. Trouble telling blues and yellows from each other is far less common, and even more rare is complete colorblindness, where everything appears in shades of black and white.

People with colorblindness can encounter trouble with Web sites or with graphics just as they can have difficulty reading traffic signals or picking out clothes that don't clash. Certain combinations of font and background colors can make the text on a Web site invisible to a colorblind person. Graphics can be impossible to decipher -- imagine trying to read a color-coded map or pie chart where some of the colors are indistinguishable from each other. Photographs can lose much of their contrast.

There is software available, though, that can help colorblind people detect the differences among colors on their computer screens. And as color graphics and displays become more complex, these programs can even assist people with color-normal vision who need help discerning shades with only subtle differences between them, such as those on a weather map.

"I think it's important" to get programs like these out to the public, says Joel Pokorny, a professor at the University of Chicago and president of the International Colour Vision Society, a group of doctors and scientists with an interest in issues related to color vision and colorblindness. Making sure Web sites and color-rich images are accessible to colorblind people "is certainly something that's on people's minds."

Tenebraex Corp., a Boston-based company that works primarily on defense technology, recently put on the market a $34 program called eyePilot, which offers several tools to help people navigate color graphics and Web sites.

One of the software's functions is a "gray" tool, which turns everything on the screen gray other than the chosen color, allowing for everything in the selected color to be seen more easily. For example, on a subway map, where lines of different colors could be difficult for a colorblind person to follow, clicking on any one of the lines will make it stand out against a gray background. Or on a graph showing the performance of various companies' stock over a given period, with each stock represented by a line of a different color, any one of those lines can be made to stand out.

The user can control how wide the definition of a color is, so that, for instance, if a graphic uses three shades of green, the program can be set to treat each shade separately or to treat all three as one color. The user also can control how light or dark the gray is in the background when a color is selected.

EyePilot also offers a "flash" tool, which makes all instances of a given color flash as white or black when the user clicks on an area of the screen with that color.

This can be particularly useful for reading maps or charts that include a color key that tells the reader what data are represented by each color. Such keys by themselves may be of little use to a colorblind person: Consider a key to a pie chart showing the regional breakdown of a company's sales. The key might show a red dot next to "Southeast," to show that the red slice of the pie chart represents sales from that region; maybe green is used for the Midwest, and so on. That key doesn't help if the reader can't distinguish reds and greens from each other. But with eyePilot's flash tool, if the reader clicks on the color dot next to "Southeast" in the key, that color will flash in the chart. Or if the reader clicks on a portion of the chart, the corresponding color dot will flash in the key, telling the reader what that portion represents.

EyePilot has a separate tool that can be used in cases where the key doesn't use bits of colors, but just names the colors. In the example above, the key might show "Red: Southeast" and "Green: Midwest." EyePilot's "name" tool displays buttons at the top of the screen with the names of different colors. So to find the Southeast sales in the chart, the user would push the "red" button at the top of the screen, and the red slice of the pie would be highlighted.

The name tool isn't limited to reading keys. The buttons can be used to find a color anywhere it appears on the screen, whether the reader is looking at a data display or shopping on a Web site. The user also can click anywhere on the screen and a box will appear telling what color is there.

The eyePilot program has one more tool: a "hue" function that changes the color scheme of whatever is on the screen. The program gives choices of color schemes, so the user can choose whichever one works best for him or her. The shifting colors are intended to make it easier to differentiate between adjacent areas that may otherwise be hard to tell apart. It can be particularly helpful for reading colored text.

Users can save and print images altered with any of the eyePilot tools.

Peter Jones, Tenebraex's president, emphasizes that eyePilot can be useful not only for colorblind people but also can help anyone deal with the increasing complexity of data presentations in color. Extremely complex models, charts and illustrations often use colors that are virtually indistinguishable, even for someone with color-normal vision. For example, world maps showing climate types or languages could use 20 or 30 colors, or even more. With that many colors, some will be quite similar and difficult even for color-normal people to distinguish.

There are a couple of free options that also may be helpful to colorblind people. One is software called Visolve, which is available from Ryobi System Solutions of Japan.

Visolve sharpens the contrasts on a screen by brightening or darkening the colors that colorblind people have trouble discerning -- it can make reds brighter and greens darker, or blues brighter and yellows darker. It can also make a given color stand out by darkening all other colors on the screen. And it can draw different hatch patterns on colors to make them easier to tell apart.

Visolve can be downloaded from www.Ryobi-sol.co.jp/visolve/en/.

Another tool is available at Vischeck.com, the Web site of Vischeck of Menlo Park, Calif.

Vischeck offers what it calls a "Daltonizing" program, named for the British scientist John Dalton, who was a pioneer of colorblindness studies. This program, which is a free Web-based service, alters images by taking colors that are hidden to colorblind people and moving them to a range of the color spectrum where they are visible again.

Alex Wade, a Vischeck co-founder, says he and his partners are in negotiations with several companies about including the Daltonizing program as a feature in hardware and software.

Vischeck also has a program that allows color-normal people to see any image on a screen as a colorblind person would see it. It's a function that could be useful to Web-site designers or to anyone who may be producing documents that will be viewed by colorblind people, but Mr. Wade says Vischeck often hears from spouses and others close to colorblind people, who like the software because they can see what it is like to see the world through colorblind eyes.