The 'old' DiffuseColor comes in 2 flavors. In v0.21.2 it stores transparency values. In v1.0.2 and v1.1 it stores alpha values. The ArchReference code should handle both. DiffuseColor is still used by BuildingParts in v1.0.2 and v1.1.
The solution is a bit hacky: if 100% transparency occurs we assume we are actually dealing with alpha values.