Hardware capabilities: display color depth

[JoJo73]

Some targets in Rockbox have same screen resolution but different color depth: this poses a problem because the values accepted by some tags (namely %Vb and %Vf) are different.

Is there a way to determine the available number of colors of the target player? If not, it would be useful to have a conditional tag like %cc which reacts to the color depth of the display (monochrome, 4-greys, full color): a general purpose theme could use it to load properly dithered images on bw/greyscale targets while retaining color pictures for devices which support them.

Thanks!