riddles...

I've decided to post the riddles I come up with here, so anyone who wants to try and figure them out can. God knows why you'd want to, but whatever. @.@

If you want to make a guess, reply without viewing other comments first. ^^


I come in all colors, black, white, gold, and green;
I do my most work when I cannot be seen.
What the force of strong men strives in vain to undo,
I, though quite small, can move easily through.
A prisoner yearns for me with all his breath,
If he cannot have me, then soon follows death.
Though thousands on thousands are called by my name,
Not even two of us are quite the same.
Who can I be? Oh, who can I be?
That is my riddle. Your answer?