I just used Minwax clear oil-based urethane on a plane with a lot of white areas (Chipmunk). I used it because I know it holds up to raw gas spilled on it. It definitely made the white areas look like "antique" white. However, out in the daylight it's not as noticeable as inside the house. IMO on warbirds and WW1 types the yellowing effect gives an old weathered look, which I like. I think this stuff is probably similar to the Black Baron clear. I tried Minwax water-based clear, which isn't yellowish, but it gets attacked by raw gas.
So...the search goes on for the ideal clearcoat that's convenient and economical.