Da Smallville Wikia :
A green kryptonite bullet can neutralize both a Kryptonian's invulnerability and their healing factor. This makes bullets a potent weapon against Kryptonians; if a green kryptonite bullet is not removed soon after a Kryptonian is shot, it can potentially kill him or her.
E em wikipedia :
In Lois and Clark, a green Kryptonite bullet is also used to bring Superman back in control when his powers become over-amped by red Kryptonite and a sample of Kryptonite was used to starve out a Kryptonian virus Superman was introduced to by Mrs. Church.
E novamente :
Green Kryptonite appears in the direct to video movie, Justice League: Doom. In the film, Metallo nearly kills Superman by shooting him the chest with a bullet made of Green Kryptonite, sending him falling from the roof of the Daily Planet and onto the pavement below. Superman's Kryptonian skin prevents the paramedics at the scene from removing the bullet, but he is ultimately saved after Cyborg uses a makeshift Kryptonite laser to cut an incision in the hero's chest, allowing Martian Manhunter to remove the Kryptonite before it kills him.
Tenho certeza de que existem outros casos de balas de kryptonita, mas na maioria das vezes é apenas uma maneira prática de levar o krpytonite para ele. Francamente, é improvável que o super-homem seja morto, por isso não veremos uma bala de kryptonita usada com sucesso nele.
O fato de estar em forma de marcador não tem nada, ou pelo menos muito pouco a ver com o método de entrega. Outros tipos de kryptonita se comportariam da mesma maneira. Eu acho que apenas o verde foi transformado em balas.