Forum Moderators: open
Check out this recent thread:
Reasons to use web safe color [webmasterworld.com]
...and this much older thread:
Only 22 TRULY web safe colors [webmasterworld.com]
I went out to a client recently to show a design, and they fired up their old Mac with Netscape 4 (that I knew about, so it wasn't a problem), only to find that my subtle color fade between the logo and the menu was an ugly bunch of stripes because of the antiquity of the screen. In that case, I explained the problem and it was OK in the end, but you need to check to make sure that your color choice doesn't make your page unusable rather than just ugly.
The other thing to look out for are for the common color-blindness combinations out there - red/green designs are a definite no-no.