There are about 15 gazillion tones of gray :-). Light gray is mostly white and dark gray is more black or other colors mixed in.
If you have not done this in art class, which I'm guessing you have not because of your question, you can take a little time to experiment by painting a gray scale wheel or series of boxes or blobs or a fan shape of tones of gray.
Beginning with white on a pallet and little blobs of other colors all around it but separated, take some white off to one side and mix in a tiny bit of black and mix it into a solid color gray...if you start with a light gray and mix toward darker then you do not have to clean the brush as much. Then experiment with tiny touches of red, then blue, then yellow, then combinations going from ligher to darker. Each time you get a new gray paint it on your color wheel or just make blotches of it...later you can cut them into squares to make a fan or wheel or just line of progressive gray from light to dark...from brownish to redish, etc.
Or, you can go to your local paint store and pick up some color cards of gray tones to find the one you like for a painting you are working on and then experiment with your paints until you match it.
Another experiment if you have the time and interest is to make a painting only in grays, like a nighttime garden scene for example...just for fun and to play with gray.
Keep having fun.