There are many ways to go about this.
You should add an instance variable for your sprites appearance: ex. Enemy.Skin.
Now when you spawn your Enemy, add the action: set value of Enemy.Skin to choose("blue", "red", "green"....)
If you plan on increasing the number of appearances, you might consider storing that info on an array and calling it by: set value of Enemy.Skin to SkinArray.At(ceil(random(0, (SkinArray.Width)))-1)
Name your animations according to the skin names you might choose ex.: "blue", "red", "green"...
Don't forget to call your Enemy's animation in the spawning process:
Set animation to Enemy.Skin
Does your enemy sprite need more than one animation to run? If so:
Change your animation names from "Running", "Shooting", etc to "BlueRunning", "RedRunning", RedShooting"
Set the animation to Enemy.Skin&Enemy.Action (Assuming Enemy.Action is the instance variable that dictates your sprites action states).
Of course, if your sprite does not animate (has only one frame) you can just add difference frames, set animation speed to 0 and randomly choose a frame to be set to.
I hope this helps!