Mix together until crumbly. Save 3 Tbsp for later. Pat the rest in 9 x 9 pan. Bake at 350 degrees for 10 minutes. Cool.
Combine sugar, rhubarb, and ¼ cup of water in saucepan. Heat and stir until rhubarb is soft and sauce-like. Mix ¼ cup water with cornstarch, then stir into rhubarb until mixture thickens. Add coloring (optional). Spread over cooled crust. Cool.
Stir together. Spoon onto cooled rhubarb mixture and spread around.
Mix together well, then pour over the marshmallow mixture. Sprinkle reserved graham crackers over the top. Chill thoroughly.