Yeah, the Knights... That yellow, it's very difficult for me...
Atia to the rescue!
Have you tried an offwhite start -> white zenithals (or I guess a fine drybrush if you have no airbrush) -> brown panel lines -> glaze of lamenters yellow -> edges with dorn yellow if there are any?
That's how I do my Legio Gryphonicus and House Coldshroud yellow!