Datatype coloring in graphical output

After #283, GDS datatype affects the color in graphical output by changing the index in the categorical color scheme. This means the `:metal_negative` layer in the DemoQPU17 example changed from a nice purple to a questionable green. It might be nice if `(layer, d1)` and `(layer, d2)` had related colors, or even the same color with different fill. I don't think Cairo gives us an easy pattern fill, but datatype could easily cycle between changing lightness, saturation, and/or blending towards black/white with different strengths.

Start by tracing the datatype-to-categorical-color mapping introduced after #283 and reproduce the changed :metal_negative color in the DemoQPU17 example. Decide on and document a consistent relationship between datatype variants, then verify the graphical output preserves distinguishable, related colors.

Written by the indexing model from the issue text.