From Sonic Retro
Knuckles has the ability to power up using the Chaos Emeralds. When he does this, he becomes Super Knuckles. Like Super Sonic, Knuckles' super transformation confers the traditional effects of a Super State which include higher speed and invincibility. Character specific traits include improved gliding and climbing. His only cosmetic change is the addition of a semi transparent fuchsia aura, which is represented in gameplay by a soft glow. It has been seen in Sonic & Knuckles and in Knuckles in Sonic 2.
Along with Tails, Knuckles was given a Super State in Sonic Heroes, thanks to Chaos Energy from Super Sonic during the last boss, Metal Overlord. While the only physical change was a golden spherical aura (similar to Super Sonic's in Sonic Adventure), Knuckles received all of the modern abilities of a super transformation. These include the ability to fly as well as the inability to brake. Additionally, Knuckles received a powerful attack where he could create explosions by simply punching and the ability to use an attack similar to his "Maximum Heat Knuckles Attack" in Sonic Adventure. This state didn't take on the original appearance of Super Knuckles and because of it, there is some doubt that it is a Super State at all.
In the Archie-published Sonic the Hedgehog comics, Knuckles can use 7 Chaos Emeralds or the Master Emerald to turn into "Hyper Knuckles" - whereas in the games, Hyper forms can only be accessed via the Super Emeralds. In this state, Knuckles's fur turns pink, and his eyes turn red. Knuckles was also able to turn into a green-colored Chaos Knuckles. This was Knuckles' most powerful form, but he lost this power when he came back from death. In this form, Knuckles was the most powerful chaos entity on Mobius at the time.
In Sonic the Hedgehog issue 141, "Hyper Knuckles" was misprinted as "Super Knuckles".