Since you wish to be a Grey Jedi, I see the possiblity for a little more freedom in the design. The Jedi have a very cut and dry look with the tunic, tabards and obi. This is supposed to be their uniform and badge of position. As a Grey Jedi, not following the Order, you could style your outfit anyway you choose. This being said, you can make use of clothing items not usually seen on Jedi. For instance, things like a bathrobe in a plain, solid color could be used as a longer tunic that would not be common in the traditional Jedi look.
Thats not distinctly true. Aayla Secura, Luminara Unduli, Barriss Offee, Tott Doneeta, Corran Horn, Ahsoka, Kyle Katarn, and most anyone from the New Jedi Order. Even Revan wore his black battle armor with the mask when he was a Jedi.
Id say that while Jedi traditionally wear robes, they have freedom. But as you said, mix and matching random clothes is actually a great way to come up with a costume, especially a non-traditional Jedi outfit. My Grey Jedi outfit was all black. It was a sleeveless vest w/ hood and shoulder mounted armor (made of plastic and metal paint), with black pants and black Jedi boots and hand wrappings. @Muulov since you want it to be obi-wan + anakins, youll probably wanna head down the traditional robe style, but as you said, the colors of your choosing. If you're having trouble coming up with a concept, Id contact Pindragon
http://www.saberforum.com/index.php?topic=11643.0