A graphical challenge

I'm planning to do a survey on my site.  Along with the usual, "How often
do you visit?", "What's your favorite part of my site?" questions, I want
to try to find out what resolution/color depth my viewers are using.  I
have a feeling that many in my audience would respond with "resolution???"
so I'd like to try to provide a bit of help.  For the resolution, I plan on
putting a graphic line, say 1600 pixels wide (won't actually be that long
due to the design of my pages, but that's another story) with markings at
each of the common widths (640, 800, etc.) and asking them to enlarge their
browser and tell me what number is closest to the edge of their monitor.
I'm pretty sure this will work well enough, but if anyone sees any serious
flaws with this, I'd love to hear them. 

The place I'm stumped is with the color depth.  Has anyone ever seen those
colorblindness tests where there's all these dots and if you aren't
color-blind you see a certain number, but if you are, you see another
number or something (or nothing).  I have this feeling that you could
somehow design a graphic that would look a certain way in 16 million
colors, then when looked at in 256, the color mapping would cause the
graphic to look like something else.  From there, you could even have a
third look for 16 colors.  (That one would be simple.  For instance, if you
made a graphic using different shades of blue in 256, all those should turn
the same blue color in 16 colors...)  Does anyone here have any clue how to
pull something like this off besides weeks of trial and error?

Thanks for the read!

Cherie
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