Though I had already implemented passing GeglColor through the PDB, it was not
complete. In particular, the protocol was not able to pass GeglParamColor specs.
Fixes:
> LibGimp-WARNING **: 16:06:09.451: _gimp_gp_param_def_to_param_spec: GParamSpec type unsupported 'GeglParamColor'
This is part of the fix to issue #10811, though it's not complete yet.