First, let me thank everyone for all the very good inputs to this question.
The use of the coloring on this first go-around is not critical, only a brown to dark brown is needed. The structure being built is a sloping hill (from a mountainside) approximately 18" high and about 5 feet long (see photo).
All the plaster-of-paris with coloring will be used for is as a cap on top of plaster gauze providing added strength and ground cover. The ground will be shaded (stained) with various paints, sprinkled with WS foam/stuff, and then the addition of trees. So, the coloring will be mostly to prevent white specks from appearing, (as the always will do) and adding to the overall color of the ground. This particular hill will have few rock formations and I will add them to the top surface.
If this all works, then I would like to use a light gray coloring as a base for my rock formations. There is always the problem of nicking the delicate detail of rocks.
Jos: Did you mean that the working time is extended (which is a plus) or that is just takes longer to dry? I'm not overly concerned with the strength part as the plaster gauze provides most of that. Anyone that has ever broken a bone certainly knows how strong the gauze can be.
Trucula: Since it is used for posters and Crayola makes it, I would assume that Hobby Lobby would have it. Also, I would guess that for a finished brown color in the
dried plaster I would need a dark brown and maybe even a bit of black coloring to come out Brown?
Rene: Thank you for your input. I am trying to work out a way to have an initial coloring of the plaster mix. What I have ran into in the past is with all the work that has to be done applying different ground covers, holes for trees, and etc., you end up nicking the plaster causing white specs. This is a real pain in the arse to repair after so many things are in place.
This is the area of work with the second photo showing a computer doctored landscape of the same area.