Add colour as a rock property

[kwinkunks]

Let users choose the colour of rocks and fluids they make.

Rocks — there's probably a scheme out there. For defaults: Sand: yellow, Shale: grey, Limestone: blue, Dolomite: purple, Halite/gypsum/etc: orange.
Fluids — Gas: red. Oil: green. Condensate: Orange. Brine: blue.

Then we can also set the colours of the default rocks etc.