"What is the meaning of the different color sashes and monk outfits?"
Why does there always have to be a meaning behind everything?
Colors are actually an interesting subject. It is a well known fact that colors are used to identify one's station or purpose in life; just ask any gang member in the Los Angeles area. We all know about wearing black for mourning, white for virgin brides, conservative browns, tans and grays to look professional, and pinks and purples to look "passionate". Vertical stripes will make you look skinnier, horizontal stripes to make you look fatter, dark colors to make you look more powerful, and lighter colors to look more reserved (and stay cooler in the summer). Red cars are sexy, black cars are sophisticated, and white cars, well, my mother drives a white car. Nuff said.
But did you know that black crosses on a door in the middle ages signified bubonic plague? Or, how about women wearing red lipstick in ancient Greco-Roman times? (It signified that a prostitute would engage in oral sex for a fee....) Where would we be without colors? Did you know that women's high heels originally started, also, in ancient Greco-Roman times? Well, it has nothing to do about colors, but, it did make it easier for the prostitutes to "position themselves" for "work". Oh, back to the colors.
Well, this has nothing to do with monk robes. Because, actually, colors have little to do with monk clothing. I've been told that the gray robes that they wear tend to be for students, though it is common to see many of the Buddhist monks wear them during their usual work days. The tan or the brown robes are used to cover the gray garments underneath; but, then again, they're also used to cover the orange or amber garments that the martial monks tend to wear. Black robes tend to signify people in authority, though their use is limited.
And as for the martial monks, that orange or amber outfit that you see, with the black sash (or yellow, or white, the colors mean nothing) with the black leg straps, has been in constant use, unchanged, for well over a thousand years.
Now, where's that girl with the bright red lipstick?