Your description of the use of class lights sounds generally correct.
First off, my personal experience was on diesels, where the class lights where built into the nose of the diesel. We either changed the one lens on each side, or on some other roads they had various color lenses permanently mounted that could be lighted seperately depending on the color you wanted to show (including red for markers, just to confuse things).
If I remember correctly, the steam era class lights had four lenses. The pictures in the rule book only show lenses directed forward and outward, but always of the same color. I am almost certain that all four lenses were the same color, if for no other reason that if they were different it would increase the risk of misinterpreting the correct color. Remember, class lights were used to help identify the train you were passing, which might be at high speed, flashing by in the dark. You tried to make certain you got the engine number and the color of the class lights (if it was displaying signals), and on those roads using them what was displayed in the indicators (train number, section number plus the train number, X-plus the engine number for extras). So for class lights to display different colors sounds like an accident waiting to happen since in some cases you would be look back at the class lights after you had passed the engine. Again, keep in mind the fundamental difference between class lights and markers. Markers just marked the end of the train, class lights were conveying information about the nature of the train you were passing.
(Note. In two subsequest posts I learned that the steam engine class lights were set up so that the front and outside facing side lenses could be lighted, while the rear and inside facing lenses were dark.....so indeed they did have both green and white (or clear) lenses, which could be changed as needed.)
JBW
