That’s something I want to know too, reader Dean. They aren’t named for the birds, I know that much. Which leaves only church Cardinals, presumably of the Catholic variety. Various entries I’ve found on cardinal slices claim that the colors of the pastry — gold, white and (at least traditionally) red — correspond to the colors of the Catholic Church. However speaking as a Catholic, I’m fairly certain that the church doesn’t have an official color palette. It has the cross — its official “logo” — and the colors of the liturgy, which is to say the various colors that are associated with specific celebrations or times of the year.
There are five official liturgical colors: white, red, green, violet and black. Each has a specific symbolism: white for light, joy and resurrection; red for blood (or fire); green for life, hope and renewal; violet for sadness and affliction, and; black for mourning, sorrow and death. To these the color rose (for joy or happiness) is sometimes added. Gold isn’t a liturgical color, strictly speaking, which is not to say it’s frowned upon in any way. By no means. Gold is everywhere in Catholicism as it symbolizes not only preciousness, but the presence of God. It’s a common color in the vestments of the clergy (it can technically replace white, red or green as a liturgical color), and is often worn on key celebration days like Christmas and Easter, Holy Days or at weddings.
Of course Cardinals are known for wearing red.
It’s the reason the birds were named for them. Red is the symbol of their office, but more than that a symbol of their willingness to die for their faith. But Cardinals wear other colors, too. Out in public they wear black, but often with a bright red cap and red sash around their waits. Celebrating a mass, however, their vestments are almost always red and white, sometimes with gold embroidery (again, especially on important occasions).
This is probably where the names “cardinal” slice or “cardinal” cake came from, at least originally. I have it on good authority that classically, cardinal slices were composed of meringue, spongecake and raspberry preserves: white-gold-red. What makes things so confusing these days is that most cardinal slices are now filled not with raspberry but with espresso cream, making it white-gold-Manchester tan (which as far as I know has nothing to do with Catholicism). I may do both versions because reader Gerhardt from Vienna would probably fly over here and beat me senseless with a buckshot-filled pastry bag if I didn’t. But between you and me, the reality is that nowadays there’s very little about a cardinal slice that’s reminiscent of a Cardinal.