Ever try duck cloth/canvas? I have seen a few kilts made out of that (Buzz Kidder) and I think Utilikilts made a workmans out of duck cloth, and I think Neokilt offers one... seems like it would be a great medium for a work-kilt... seems like it's tough stuff. Looking good on your kilt.
A nice black tweed would make a fine solid colour for fancier occasions in my opinion.