I did the same.
Got my 9 chickens to cough up 7 eggs.
Hard boiled them.
Then cracked them.
Then dripped the coloring right on them to capillary into the cracks. Worked great!
Then peeled them.
Of course having three separate food coloring kits and not a one had red in it. {sigh}

Unfortunately everyone around here said things like, "Ugh what happened to them?". Instead of, the more desirable, "Oh how cool!", I was hoping for.

My raku eggs didn't turn out good this year.............But hey it was fun. I couldn't get a photo that adequately shows the metallic color...sorry guys! :)