I already have two bottles of MM enamel olive drab. I just bought a set of "camo color" set from tower hobbies, that comes with 6 different colors including olive drab. However, the olive drab that came from the set is greyer" than the two olive drab bottles I already have at home which are a little greener. Are they the same color? or there are two different tyep?

There appear to be as many different shades of OD as there were vehicles painted with it ! - USAF olive drab was different to US Army OD, and British OD (khaki drab) was different again!
In a major article in MM some time ago Steve Zaloga concluded that Tamiya made the nearest to US Army factory-fresh OD. As the original colour was mixed from yellow ochre and black almost every mix seems to be different from the next, and the same applies to model paints - I have Tamiya, Humbrol and ANother acrylic paint - all are different so I use that as a way of getting variations between vehicles.

there are two different shades of OD. One for vehicles and one for aircraft.

Easy answer is yes. And this is not just a problem with model/hobby paints. Just as it is/was with actual military colors, each manufacturer uses a slightly diferent paint mix to get Olive Drab, or any other color for that matter. Also, depending on the year it was manufactured, the paint can vary slightly as well. This is seen onvehicles as well. Ones that came from diferent plants or from the same plans at diferent times can have a great variation in paint shades, even though it is the same color. Also, vehicles that are in service will fade and weather at diferent rates, changing the color even further. So, yes there are diferent shades of OD Green, and it is a good thing.

WW2 OD and post war OD are different, with the post WW2 OD haveing more black in the mix.

As you can tell there are at least 2 kinds of OD. I don't mind as it give variations in my OD tank collection mixed in with OD/black camo patterns

so the sherman tank used the "darker" shade of OD? how do I tell which OD is which by there "numbers" cause the darker OD I received has its number smured.

It really doesn't matter which OD you use. Pick the one that pleases your eye.

I think MM Green drab is the late od color. I use it on all my Nam era stuff. Korean war I have done a 50 / 50 mix of OD and GD for a nice variation.

uh....two kinds...er.....wet and dry