Extra colors

This implements a solution to issue #261 by using a nested cell.
I noticed the color definitions were much too detailed for any practical use. In the output, I limit the definitions to 6 decimals (60 bit color space) instead of double precision (15 decimals or 150 bit colorspace), while e.g. Photoshop is limited to 32 bit RGB.
So it is still overkill for practical use, but a lot less. The definitions look nicer that way as well :-)