3getting a pair
Roller Skates are stock items rather than crafted ones, sold by the Traveling Merchant and the Skeleton Merchant when they feel like it. The four colors are cosmetic. Whichever one turns up in the rotation is the one you get, and nothing about the ride changes between them.
how they change your movement
This is a momentum mount. Once you are rolling you keep the speed without holding a direction, and you only slow to a floor rather than stopping. That floor is tied to your block placement speed, so building buffs raise it: a Builder Potion, a Brick Layer or one of its upgrades, or having eaten Ambrosia. The Brick Layer only counts while you are holding a placeable block that is not a torch, and stacking two of them does nothing extra. Jumps get taller and hang longer, well past what a Frog Leg gives, without eating an accessory slot. There is no fall protection attached, and you also drop faster, so a long fall hurts more than usual.
what they combine with
The reason to use these over a faster mount is that they do not take anything away. Hooks, wings and dashes all still work, and a Cloud in a Bottle or its upgrades still fires. You also stay three tiles tall, so doors are not a problem and you can skate straight through your own base. Hovering wings gain the most: because the game reads you as coasting rather than idling, a Hoverboard, Celestial Starboard, Vortex Booster, Nebula Mantle or Betsy's Wings burns flight at the reduced rate while still moving you at full hover speed, roughly doubling how long you stay up. The exception is Terraspark Boots: their speed and flight bonuses do nothing while skating, though the lava wading half still protects you.
where they earn their slot
They ride Minecart Track, which makes building a Wall of Flesh run almost trivial: lay the track through the Underworld and skate it instead of laying a proper cart line. They also flatten slowing debuffs like Chilled and Mighty Wind without granting formal immunity, so a Sandstorm or a frozen cave stops mattering. The trade is that all of this is momentum, and momentum is bad in a boss arena where you need to reverse on a dime.