Broadly speaking, apart from some minor modifications as pointed out by Ben and others, US-manufactured vehicles were pretty much used as they left the factory, including the Olive Drab paint.
Before the US was engaged in WW2, the British and other Allies has more room to specify their modifications. The M3 Grant tank, for example, had a completely different turret fitted to the hull of the M3 Medium Tank as the US Army specified it. When the USA was fully engaged in the war themselves, it was basically take it as is or leave it.
Regarding unit markings, Commonwealth unit markings differed vastly from US ones.
On the subject of recognition markings (roundels and stars), I quote Don Dingwall's CANADIAN ARMOUR IN THE ITALIAN CAMPAIGN ITALY 1943-1945, Canadian Vehicle Markings, Order of Battle and War Establishments for 1st Canadian Corps and 1st Canadian Armoured Brigade:
"By the spring of 1944, the U.S. star was adopted as the standard AFV recognition marking. Prior to this, the combination of the red/white/red AFV flash and a roundel of red, white and blue, similar to early war RAF roundels, was used. The roundel was applied to the tops of turrets or engine decks on Shermans and other vehicles where space allowed, and on the engine hoods of Otters and other wheeled AFVs, and on the cab roofs of many softskins. This was officially discontinued on April 11, 1944, but roundels were still seen for some time afterwards."
See "Crooked White Star" on the MLU Forum (http://www.mapleleafup.org/forums/showthread.php?s=&threadid=4760) for the official Canadian instructions to mark vehicles with "White stars five pointed". There is no mention between stars in broken or full circles – this was the difference between stencilling and painting by hand! It does give an explanation for the “crooked star”: the instructions included a drawing of the star standing on one end!
Hope this helps,
Hanno