As promised, I was able to prepare some photographs of the numerous Chests I have in my collection which still retain their original paint. As you will see, there is a huge variation in the colour and finish used, and this likely reflects time period and manufacturer.
First, is the MD4 Chest. The contents are painted in a satin-finish Olive Drab. The markings appear to have been applied with stencils and a sponge (white lusterless paint). You can see the interior colour of the Chest in the top left corner of the photograph (matches contents):
Next, is the MD1 Chest, complete with inserts. What is interesting here is that the inserts have been painted OD, while the table top and Litter Supports are painted lusterless black. The OD used for the inserts is far greyer than that used on the MD4 Chest. I also removed the bottom section of the lid to illustrate how these were painted:
Finally, here is a Plain Field Chest, which has had the lid removed. It shows the interior colour, and also the exterior colour. You can see underneath the hinges that both colours were identical when the item left the factory. The exterior now appears much darker, but this is likely as a result of natural weather over the years. You can also see the yellow primer that seems to have been used on the Chests (also present on my MD4):
I hope that the above might help to answer your question regarding the 'correct' colour for the Chests.