How "old" are we talking? Jack Delano took countless color photos of workmen (and women) during WW II and they are available on line. I know many of them wore blue denim but believe there also were many in canvass-colored brown.
If you want to get serious, I'd do a search for the corporate history of some of the obvious garment-makers and see if they tell what year they began business. They may even have pics of some of their early products. IIRC, Levi Straus began making garments for miners during the California gold rush. Also check Carhartt, Lee, Oshkosh, there are probably others.
But for a quick answer, either tan or "blue jeans" color. Look at the contrast in your friend's B&W pic and see what shade they seem to be. Side by side in a B&W shot I'd expect tan to appear lighter than denim blue but it would depend on wear, grime etc. At least nobody is going to still be around to say you're wrong.