They were sent out in "factory" colour's,green/brown,or grey.
When in theatre it depended on what paint was in stores,and or the crew,also how fast they were deployed.Going into combat in the colour they arrived in.
If plenty of paint was to had then a sand colour was prefered.If supplies were limited then two tone schemes of either stippled or solid spot/blotches,over the base colour. Or a wavey lines in "Y"shaped patterns again with the base colour showing.
Both schemes can look scruffy in scale though. You can't really get it wrong,as it often came down to the skill of the crew.
In short paint it the way you feel looks right for your game,as long as you like it and your friends don't mind it on the table ,then don't worry,as someone will always tell you your wrong no matter how much research you do.
Mark