I use CS3 mainly for a coloring. For a mild I'm going to look for something that will provide more of a nutty and to a lesser degree roasty flavor. This would include the lighter roasted malts like the chocolates, light roasted barley and coffee malt.
Now if I want to darken it up, then I'll add some CS3. Actually that might be when I cheat and break out the brewer's caramel coloring.