So I had a go at the xcf file and this is what I've done. I used the color select tool on the colorized versions. You want to be sure it is set to 'add to current selection' and then I clicked all over the colored parts that I wanted to turn black. Once I had it all selected I painted over it with the paintbrush. It's like coloring in a children's book only with better guides for the sidelines. (Hehe, I am not good at coloring in the lines.) I cleaned up some of the fuzzies on the sides and where it needed it I painted eyebrows back in with a lighter grey. I noticed the original renders are missing a couple of pieces like the foot was cut off on some. You might want to paste one of the not cut off feet in place there before moving on.

Hope this helps.

Very nice job EK. I didn't think to click the add button, good idea. They are a bit fuzzy around the edges but maybe that could be fixed with a small blur.

Or the eraser tool and a soft brush. I figured I'd let you work out the details though since this is your baby.