Add a default value input for geomcolor

It would be nice if Geometry Color could specify a default value, so that users can choose what color to provide when the geometry property is missing.

Currently, many people just use Geom Prop Value, because you can specify the default color.

Thanks!

Start by locating the Geometry Color and Geom Prop Value implementations and compare how their inputs are defined. Determine where Geometry Color handles a missing geometry property, then add a user-selectable default color and verify that the default is used when the property is absent.

Written by the indexing model from the issue text.