How can I change the bits-per-pixel in X?

The easiest way to do this on a regular basis is to configure X properly. However, you may want to set the bits-per-pixel the old fashioned, difficult way. Try:

startx -- -bpp 16

to start X in 16-bits-per-pixel mode; other modes are 2, 8, 15, 24 and 32. Your server may not support specific modes, so your mileage may vary.

See man startx and man xinit for more information.

You can also set DefaultColorDepth in XF86Config (for XFree86). This file will be in /etc or /etc/X11. Inside that file, locate the relevant Screen, and insert it somewhere inside the screen section, but outside the display subsection. Note! There could be more than one screen section defined! You must be careful to choose the right one.