This is a challenge.
A two-color version requires and permits movement after taking one's position.
So I don't think it provides useful guidance for this case
Spoiler for here it is anyway
Eager to see the solution.
Spoiler for solution
I don't see the OP requiring each prisoner to assume his position before the next one enters the room. So why should we be adding such a restriction?
On the other hand, the OP never specified that the place for each row has been marked with like colored chairs, tile, or in any other way. Thus we must not assume that it has.
Given the above, Rainman's method needs just a slight modification.Spoiler for how about
As for the blindfold, it is kept on until each captive enters the hall to prevent them from seeing the color when it is placed upon their heads and thereafter to hinder them from checking the same in the mirror of the dressing room.
Hasn't Rainman already solved the puzzle? With that small modification I added, so that his solution does not rely on the preset place where to form rows.