You mean people coloring their armor black and red? Actually I don't see many people doing this.
But on another note Black and white are the 2 most desired colors in any armor color. Red and yellow are the 2nd 2 most desired. Is it any supprise that people would use colors that are looked on as great and wanted/desired by many?
My armor was necro blue and lamae white for a while, with a bit of black mixed in for detail. I recently redid it to a more black/red color scheme solely to match my main guild's tabbard, which is black with a red rose. In my case, at least, it's more about "this outfit matches" than "OMG DAEDRIC", and I still try to mix in other colors.
I see more shades of green than any other color. I'm trying to figure out which ones have a shiny gloss and which ones look matte so I'll do a range of colors, go play a bit then change it again.
Unfortunately we cannot dye shields and weapons yet. Until then you have to match something. Not like you can go out with your socks a different color than your belt, ya know?