Hi STL:
Colour depends alot on the campaign;
Europe and Pacific, you will see a lot of OD. North Africa, sand and tan shades.
Often you see photos of Shermans in the field, so they are a grimy weathered lot, and that can translate into grey via photo and printing.
If you can, spring for FSM's "Building and Detailing Realistic Sherman Tanks." -yeah, full disclosure, I get $50 for each one I sell.
It mentions the use of grey in some Pacific colour schemes. Once you know exactly where and when your type of Sherman served, you can usually find reference photos or built up examples to help you with the colour.
There are quite a few reference books available as well. But I only get a $35 commission for those!
Many a builder spends as much or more time reading/researching than building, history is a huge part of the hobby.
Keep us posted.