Indexed Color Mode

I need to read back from the back buffer the colour index values, I’m using the glReadPixel command, but while it works perfectly with non indexed color modes it doesn’t work with indexed modes.
glGetError returns no error, but the int buffer that should receive the indexes remains unchanged.
Do someone knows where to find some indexed color mode samples and glReadPixels samples using indexed color mode (seems noone is using it anymore) ?
Thanks in advance.