M. Graham & Co. makes a line of professional oil colors with a walnut oil vehicle, as well as walnut oil mediums. The paint is beautiful buttery stuff. Those enquiring here about color charts will find one at the Graham site, under "Oil Colors".
A cautionary note -- the paints come in tubes with unusually wide mouths. You have to pay attention or with just the typical squeeze you wind up with a tablespoon of butter on your palette. For those of us who have the fault of using too little paint, this is perhaps not an altogether bad thing.