In the comics, Kryptonians' heat vision is typically red — so why is Supergirl's heat vision blue in the Arrowverse? When Superman made his debut in 1938, he was very different to the character today's fans know and love. Even his basic powerset was remarkably different, as proven by the gradual addition of his heat vision.
Originally, Superman had some sort of telescopic and x-ray vision. The first hint his vision could be used offensively was in Action Comics #59, when he melted a glacier "with the tremendous heat of his x-ray vision." Over the decades that followed, Superman used this power to dramatic effect, even melting a hole all the way through the Earth in Action Comics #298. Little by little, writers began to treat this as a separate ability rather than an extension of x-ray vision, and colorists picked the color red for this so-called "heat vision." But the Arrowverse has taken a different approach.
In the Arrowverse, Kryptonian heat vision is actually blue. No official reason has ever been given, but the most likely explanation is that a blue flame is hotter than a red one, so this actually emphasizes the intensity of the heat. To give an idea, here's how hot a flame of a different color burns:
- Dark red: 930-1,112 degrees Farenheit
- Dull red: 1,112-1,472 degrees Farenheit
- Bright cherry red: 1,472-1,832 degrees Farenheit
- Orange: 1,832-2,192 degrees Farenheit
- Bright yellow: 2,192-2,552 degrees Farenheit
- White: 2,552-2,912 degrees Farenheit
- Blue: Over 2,912 degrees Farenheit
Of course, in reality a Kryptonian's heat vision is not a flame, so the comparison is inexact; still, the fact remains that the red color denotes a less fearsome temperature. As artists became aware of this issue, they gradually began adjusting the color to add more yellow, emphasizing the growing heat; this was typically used in scenes where Superman was exerting great effort, suggesting he had to strain to increase the strength and temperature of his heat vision. There have even been a couple of cases in the comics where he achieved white hot.
But the Arrowverse evidently decided to skip all that, and instead emphasize the heat of Supergirl's vision by turning her beams a bright white-blue. Interestingly, some scenes have mirrored the comics in suggesting using heat vision extensively depletes Kara of her own energy, which makes sense; her body is essentially a giant solar battery, and it can only hold so much. Emitting such a powerful surge of heat would naturally drain it dry. Although the change is a departure from the comics, Kara's depiction in Supergirl makes sense — and follows a scientific logic modern viewers can appreciate.