Item Skins, as the title says are alternate appearances for Items which can be obtained through different means. These give the Items different appearances when playing as the Survivors.
The glowsticks handle turns a blueish color, while the light turns a glowing light blue. This is given when the player reaches Diamond in the Levels tab with the Glowstick.
The Doomfist loses the gray thing on the front of the gauntlet, and replaces it with 6 different coloured stones, one purple, one blue, one red, one orange, one yellow and one green. This is a direct reference to the Infinity Gauntlet from the Marvel franchise.
The gauntlet mainly turns a shade of blue, with the spikes being a lighter metallic shade and the lights being a glowing white blue. This is rewarded when the player reaches Diamond in the levels tab with Doom's Gauntlet..
The Weird Mask's eyes turn bronze. This is rewarded when the player reaches LVL 10 in the levels tab with Zombie (and for extension the Weird Mask).
The Weird Mask's eyes turn silver. This is rewarded when the player reaches Silver in the levels tab with Zombie (and for extension the Weird Mask).
The Weird Mask's eyes turn golden. This is rewarded when the player reaches Gold in the levels tab with Zombie (and for extension the Weird Mask).
The Weird Mask's eyes turn light blue. This is rewarded when the player reaches Diamond in the levels tab with Zombie (and for extension the Weird Mask).