The RES's shell is red because it is stained with iron ore from the environment. That will go away in time. Especially after the annual lamina shed. I'd keep him in a smaller container and offer him chopped earthworms or chopped small minnows to start. Then get him on the prepared foods. Then move him to the larger tank.
It's not unheard of for a turtle the size of the Painted Turtle to accidentally injure or kill a small one like him at feeding time.
jsc
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"As hard as I've tried, just can't NOT do this"
John Crickmer