You may have been told that graphics appear darker on a PC than on a Mac. This is only a half-truth.
The colors in the Web palette are 20% steps of Red, Green and Blue and are shown in the chart opposite. Ideally, you should see five shades of each color, the sixth, (0%) is black in each case.
If the 20% shade of each colour looks black too, then the monitor is not responding in a visibly linear fashion. The relationship between RGB values and perceived brightness is determined by the 'gamma' of the monitor. On a typical Mac screen you will see a good separation between 0% and 20% values of RGB. Mac monitors are generally optimised for print work and have a default gamma of 1.8. PCs do not usually have this gamma compensation and have an 'uncorrected' gamma of about 2.5.
So what is correct?
The answer is neither. There is too much of a step between 0% and 20% on a Mac and too little on a PC.
The W3C, in it's wisdom, recommends that Web designers work with an intermediate gamma value of 2.2 which is a compromise between Mac and PC gammas, but is also the standard for broadcast television. If you work with a gamma of 2.2 for Web design work, there will not be such a dramatic difference in what you see on a Mac screen and what a surfer with a PC sees. Luckily, it is easy to set a Mac monitor to a 2.2 gamma through the Monitors and Sound control panel, most PCs have no such option.