Killer Instinct

The easiest skin to unlock is Skin Color 3 (1 and 2 are unlocked by default for all characters thus far). In order to unlock this for purchase you must unlock 50 fight titles for Spinal. This can be done simply by fighting in any mode (versus, solo, ranked, exhibition). There are no character specific fight titles for practice, but there is a category for minutes spent in practice in general fight titles. This will not help you here, just thought I should mention it. Once you have unlocked the 50 fight titles (also netting you the Spinal Novice achievement for the first 30), go to the Ki store and use LB/RB to reach the skins tab. Scroll all the way to the bottom to find Spinal's and purchase the newly unlocked Skin Color 3 for 1000 Ki Points (Easily built up during your collection of the 50 fight titles).

As for collecting the 50 titles, I found survival to be the best, as you can get the Survival Spinal and Spinal's Endurance achievements for your first and tenth wins. Set the difficulty to beginner to make this even easier. It is also possible to get the Spinal's Searing Skull achievement if you are pitted against Spinal in one of your battles. As Spinal, against Spinal, if you go into instinct mode, cast Searing Skull on the CPU Spinal and if it hits, another achievement is yours! If you want yet another one, do a single Versus match before starting the Survival grinding. A single Versus victory will net you the Sparring Spinal achievement.

Colors for each character unlock at Level 3, so go into Survival or any of the fight modes and win 2 or 3 matches, preferably doing some of the fight challenges associated with each character

Spinal earns a new color after lvl 3. You'll get enough XP for this while passively playing 200 matches. Idling is good enough for this achievement, but you will earn it faster while going for 10 survival wins.