I use GW paints for British scarlet - a base coat of Red Gore, followed by a first highlight of Blood Red, then Blazing Orange (or the old Blood Angels Red if yoou can get your hands on it - it's slightly more "red" than "orange") and Fiery Orange for the final highlight (usually watered down quite heavily, unless you want a really faded scarlet colour). For blue, I use the Foundry "Deep Blue" 3-colour pack or, in the GW range (and for a slightly lighter blue finish), Regal Blue followed by successive highlights of Ultramarine Blue and Hideous Blue. I suspect the blue required for Royal Artillery should be much darker, but for simplicity's sake I paint all my AWI artillery crews in the same Deep Blue, whether Yank or Brit.