You might look at using "web safe" colors. There's a small selection, but they survive intact with lower-bit color resolution. You might do ok if you're after greys.
Another problem that arises is that browsers render colors differently if they're not web safe.
I had the same problem, I think, with the dark blue on
this page where the banner across the top overlaps the left column. The banner's dark blue is from Photoshop and the color in the column was from Dreamweaver. I had to just juggle them to get them to match, playing with each color to get it right. I couldn't get it to look right in Safari, Firefox and Internet Explorer. It would look OK in one, but not the others, so I went with IE since it's the most widely used (arrghhh). The numbers don't match, but the colors sorta do, unless you look at it on an LCD screen at a high angle. I'm probably the only one that notices. I couldn't find what I wanted in web safe colors.
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now known around here as Pat Foster
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http://www.patfosterguitars.com