Refactoring of ColorLogic (part IV)

Since rectangle fill values were pulled from environment view to view
model, trying to update the fill brush from the "tick" update thread
causes 'must create dependency source on same thread as dependency
object' problems, so moved away from storing 'freezable dependency
properties'
(http://stackoverflow.com/questions/7865514/must-create-dependencysource-on-same-thread-as-dependencyobject/7865548#7865548)
in the view model (now storing Color instead of SolidColorBrush)