If the color of cast iron is a reddish color on the inside of a coal stove does that mean the stove was over-fired or over heated at one time ??? or is this normal? the cast iron is not cracked or warped that I can see.
I beleive that is normal oxidation from the sulfur and moisture in the air over time. Thats why cleaning is so important after the heating season. Some folks spray oil on the inside of their fireboxes after cleaning and prepping their stoves for the summer humidity.
Normal, BTW when you clean or do anything within side the stove be sure to thoroughly wash your hands. It's along the lines of handling hot peppers, you don't want to accidently get any of that in your eyes.