There should be a bit of red beneath that slate as you get closer to the skin.
Actually, that's not what the Standard calls for. The Standard says specifically "UNDERCOLOR OF ALL SECTIONS: Red, except for back which should show a bar of slate."
And yes, it's lovely if there's some red at the base of the feather, but strictly speaking, the Standard does not require that. It just requires that the bar of slate should be present in the back is all.
See the link in my signature for the SOP for large fowl Buckeyes.