Ever finish a diorama and get a ding in it only to reveal white plaster? I’ve had it happen to me.
Had I done a little more thinking before a little more doing, I could have avoided having the white chips of plaster showing through on the base.
Mixing matching-color water based paint in with the plaster would have alleviated the “show through” problem.
With a little planning, you could mix several different color batches of plaster depending on the area that your diorama takes place. For instance, Mississippi has a lot of red clay; the Delta has alluvial soil. That would suggest reddish base color where there’s red clay and a blackish tone where there’s alluvial soil.
Sure, I could go back and touch my masterpiece up, but it sure would have made my life easier if I had tinted the plaster in the first place.
That way you’ve got time for things other than doing touch-up work. After all, there are always more models to build and more dioramas to create.