it is no untypical - not only for Lejeune, that military artist - even contemporary ones - did paint the uniforms of the time when they executed the painting and not the uniforms which were worn at the actual battle.
Also of course when the artist what not present at the battle at all - he had to use imagination, Lejeune did paint very detailed - and also quite fitting uniforms for the actual battle as well, it would depend when he would do it, but they need context.