Is it better to apply the blue metallic nail powder with a sponge or a brush?

It is better to spread the turquoise metallic powder with a sponge. When you buy TGB Aqua Chrome Pigment, it comes with a nail sponge (applicator). You can rub the applicator with a lint-free wipe or paper towel to remove excess pigment or wash them, then save it for the next use.