Here in the US many judges are not clear on a dark factor slate. and many breeders of slates do not use yellow face or cobalt violet in the breeding to make the most common sky blue slate as clean as possible. We currently have these other slate types in our aviary and we are working to see how well we can produce each distinct color of slate. Some of the rare breeders here in the US are talking about using slate to darken the anthracite.
Blessings, Jo Ann