Define "preferredColor" for observed property categories or remove from playground example

[letmaik]

The playground example "Grid (categorical)" includes "preferredColor" for each category, however this field is not defined in the spec. It's supported in leaflet-coverage.

Either the spec should define this field or its use should be removed from the example.

See also:
covjson/playground@2053965
Reading-eScience-Centre/leaflet-coverage@b3ddd74