It also depends on what particular unit and time frame you want to model. Some units, like the 1st, stayed with leggings and wools through 1945, while other units, like the 44th, switched to HBTs with double-buckles and M36 packs.
Khaki was worn by most GIs right through 1945, and it was not unusual to see a mixed variety of OD and later OD7 (darker than the previous) gear issued too.
Tankers often wore khakis, but they also wore HBTs in OD, and the M41 jacket is seen in both khaki and OD, although the M43 jacket is only OD.
In some cases when surplus WW1 gear was issued, it was an even browner khaki than the WW2 variety, especially with cartridge belts for the Garand and BAR webgear. Course, these were also issued in WW2 khaki and OD too.
I guess you can pretty much make it up as you go along. It's not uncommon to see a GI with khaki webbing, OD musette bag, leggings, and new, dark, OD7 Carlisle pouch.
Let me know (by email) if you need any help. I'm a collector of US WW2 gear and have about 250 items in my collection.
Jeff