Monique, my advice is that you will need two different type/style bras.
Keyhole top: I suggest a “plunge” style bra; that way the material that connects between the cups will not “peek through” the keyhole. This style also works for some “V-shape” necklines. I’m wearing a “plunge” bra in “My Beginning Style”, “Debut Dress”, and “Classic Pencil Skirt” photos on my Public Photo Page. Feel free to have a look.
Low-cut top: I suggest a balcony, “demi”, or shelf cup type bra; that way the material at the top of the cups will not show above the neckline, unless you want it to show. Otherwise you might need to layer a cami or tank top underneath.
I hope this gives you some direction; Brittney.