I imagined them as red as well!
I also imagined the 1910 dotted skirts-and-suspenders to be red, and I just compared the gray, and it does indeed seem like they were red!
I wonder, if someone took a bunch of fabric in all different colors, and laid it out with labels as to what color was what, and took a picture of it in black-and-white, then we could roughly tell what color OTMA's clothes were!
Maybe I'll do that, and post it. It would be an interesting experiment.