For all the many variants of Supergirl's proved that one is surprisingly the most effective. Green may be the most common version, but a lesser-seen type can be the deadliest when in the wrong hands.
After Krypton's devastating explosion, shards of the planet were scattered throughout the universe. These little pieces of Superman and Supergirl's home world were transformed into kryptonite and became one of the most widely known Achille's heels for Kryptonians. The most classic version of the rock comes in its green form and exposing Superman to kryptonite could have deadly effects. But green is just one type of the radioactive meteorite. Kryptonite has also come in other forms such as red, gold, and various synthetic variants. What makes kryptonite such a threat is that each version has it's own effect on Kryptonians, with some being being much deadlier that others.
Supergirl's occasionally recurring nemesis Reactron made a strong argument for which form is actually the strongest in Superman #683 by James Robinson, Renato Guedes, and Jorge Correa, Jr.. As part of a covert military effort, Reactron is drafted to take out Kryptonians that have taken residence on Earth. He’s partnered with Superman’s kryptonite-powered villain Metallo, who uses his trademark green kryptonite heart to do some damage. Reactron, on the other hand, is given a gold kryptonite heart. While it's not as lethal as the green variant, gold can depower a Kryptonian for fifteen seconds, or as Reactron says "plenty of time to take care of business". After stripping innocent Kryptonians of their abilities, Reactron brutally slaughters them with lasers. He even manages to expose Supergirl to gold kryptonite, though her father saves her at the cost of his own life.
Gold kryptonite used to be more powerful and could actually depower Kryptonians permanently in DC’s old continuity. But Reactron seems to get along just fine with fifteen seconds. There's no argument that on its own, a piece of green kryptonite would do more damage to Superman or Supergirl simply because of its intense effects. But in the hands of a capable killer like Reactron, gold kryptonite can be used to a much more lethal degree.
When a Kryptonian is exposed to green kryptonite, they experience so much pain and weakness it practically renders them feeble. But Superman has faced plenty of criminals that wielded the standard kryptonite before, it’s not exactly a challenge he’s unfamiliar with. However, Supergirl’s villain shows there's very little a Kryptonian can do if they come face-to-face with gold kryptonite. They moment they realize they've been exposed, they're powerless and if they're powerless, there's nothing protecting them from anything that would kill a human. Bullets, knives, lasers or any manner of superpower could effectively end Superman or Supergirl's life under the right circumstance.
Green kryptonite's only real advantage is that with enough exposure and time it could kill a Kryptonian. But as Supergirl's father found out, gold kryptonite provides a window of opportunity for a skilled killer to make their move.
