Not sure how it worked in the 20th Century, but in the 19th, how landing parties were decked out seems to have been at the whim of individual ship captains. The idea was you had Tropical whites and Temperate blues, but this seems to have been ignored much of the time. The landing parties from different ships in the Zulu War were all dressed differently to each other, some all in blue, some all in white, and some with blue tops and white trousers.
So unless you are recreating an historical engagement for which the uniform colours are known, you could get away with dark blue uniforms if that's what you feel like painting instead. If you do prefer that option, the collars would be the same colour as Carlos describes, much lighter than the very dark blue of the RN uniform.