Atheism is lack of faith. This doesn't mean you stand between the choice there is god/there is no god, you are quite literally lacking the ability to answer, and frankly you don't care.

Agnostics assert that it's impossible for human beings to know anything about how the universe was created and whether or not divine beings exist. It's about uncertainty, not lack of faith.

You're oversimplifying atheism and agnosticism. There's some overlap, particularly with how you define atheism as implicit "weak"/"soft"/"negative" atheism (which somewhat overlaps with apathetic agnosticism) and agnosticism as strong (or hard, closed, strict, or permanent) agnosticism ("It's unknowable to me or anyone else").

There's also explicit strong and explicit weak atheism ("At least one deity exists" is false and a rejection in belief deities exist without asserting the previous statement), as well as weak agnosticism ("It's unknown whether any deities exist, but it might be possible to know").

The definition of apathetic agnosticism ("I don't know and I don't care about the existence of deities") and implicit weak atheism ("I have no concept of a deity and thus no belief in one") is similar, but not necessarily identical.

I also don't understand your metaphors (which appear to mix existence and color):

"Your car is blue"

"No, we cannot be sure it's blue, it could be red or blue. I don't even know if I own a car"

"I don't own a car".

In the first case, I assume that's an example of theism.

In the second case, is this your description of agnosticism? Because it's not clear if you're saying this is strong or weak agnosticism (strong agnosticism, as you defined agnosticism, would be "No one can know what color my car is", while weak agnosticism would be "No one knows what color my car is but it might be possible to find out someday"). Apathetic agnosticism, which seems to be how you defined atheism, would be "I don't know what color my car is and I don't care." This could also tie into "I don't even know or care if I own a car."

I'm still unclear as to how agnosticism determines if a belief is an individual belief (I don't know) versus a group belief (I believe that no one knows).

In the third case, I guess this is atheism? But this is an example of explicit strong atheism ("The car I own does not exist") versus apathetic agnosticism, which is what you described ("I don't care whether I own a car or not").

Basically, your definition of atheism is apathetic agnosticism, while your definition of agnosticism is the definition of strong agnosticism. You're excluding people who explicitly reject the existence of God, and also miscategorizing a LOT of (apathetic) agnostics as atheists.