Perfectly normal, typical loach behavior. When my clowns tussle they go from being bright orange & black, to dull orange and gray. When my yoyos tussle they turn so pale you almost can't make out their markings.
When I first got my group of yoyos I didn't know this and did exactly what you did. I was convinced that the whitish looking loach was sick and needed to be removed from the tank. I tried chasing her around with a net, to no avail. I decided to give her a break and come back to it later. While on the computer and doing research I find out that it's healthy loach behaivor. Then, of course, I felt bad for torturing her for nearly 30 minutes trying to catch her.
How many zebras do you have?