Run screenTest on windows98 or windows2000, set the color depth of the primary
screen to 256 or 16 bit, set the color depth of the second monitor to 24 bit or
higher. You can tell that images on the second monitor are drawn with the color depth of the first monitor by watching the color dithering.

The reverse is not true. It can be used as work around: set the color depth of the second monitor as 256 or 16 bits, set the color depth of the second monitor as 24 bit or higher, you can get the color depth you want to use.