Digital persistance: apparent intensity, RGB values

I have a color display and I want to make a device that utilizes digital persistance, that is, that the apparent intensity of a pixel is proportional to the time(s) that it is set. My problem is that I am having trouble coming up with a table of RGB values that looks "right." I know I'm not the first person to do this, so does anyone know where I could get such a table? Ideally it would be 256 colors. If I could get a *good* bitmap of one, maybe I could use an eyedropper tool to build a table...

j.