My first piece of advice is to simply call or e-mail to clarify. If you can't get in touch before your workshop, I would absolutely stick to black pants and black non-skid shoes. You can definitely save black pants for later use. A good pair of heavy black work boots are also invaluable in case you do any kind of heavy duty work later. I would also wear a plain white tee shirt with perhaps a sweater or button-up blouse over it. This way, you won't look so foolishly overdressed in checks if everyone else is dressed down. Also, in case they give you some kind of jacket, throwing it on over a white tee would be no problem. Also, if they don't give you some kind of jacket, white is a very traditional color for a French kitchen. Even if you aren't doing a stage or looking for employment, my feeling is that you're their guest no matter what, even if you are paying a big sum to be there. If anything, maybe you can just bring a change of clothes?