Your client sized his browser window so he can see crucial areas of his desktop, which means his browser window wasn't as deep as yours. Similar glitches crop up when you design only for your monitor size. With a rigid Web layout, your site may appear to be shoved into the corner of a visitor's large monitor. But if you design specifically for that space, your site may look ludicrous on a larger monitor at 1,600 by 1,200 pixels. The answer is to embrace the fluid nature of the Web so that content reflows according to visitors' operating systems and window sizes.
Different platforms have different standard gamma settings.
Put simply, your Mac's default gamma of 1.8 looks brighter than the various Windows defaults. Ignore this difference, and your subtle earth tones will look like mud. If you don't have these programs, two low-cost shareware apps and the following steps can help. One app is GammaToggle. If you have older versions of Photoshop, you may want to use Furbo Filters.