(I'm going to assume you mean spawning an NPC in Icon, and then using the load costume command to change them into the required NPC.)
When the game was up and running, yes you could indeed recolor most enemy NPC units in the game be assigning them into a custom enemy group and altering their costumes through a costume creator-like menu. It didn't always work as expected depending on the enemy and the costume parts in question, but it did allow you a certain amount of freedom. You weren't able to re-color their powers though.
Now, in order to spawn one like that, you'd need a costume file saved with the requisite information. You MAY be able to take a look at a saved enemy group in MA that had custom colors and try to fit that formatting into what is in a .Costume file... but that'd take actually going in and editing the file in a text editor of some sort and then re-saving it as a .Costume file. Then, you'd have to try it out.
You'd probably litterally have to go line by line in the custom enemy group file and look through exactly how the parts are documented and then HOPE that it can be matched up to the formatting of a .Costume file. Man, that'd not be fun.
I very much doubt that it'd work, or be easy... but it's worth a shot.