OK, first, a simile is a comparision and will compare an object to another object, using "like" or "as".
A metaphor says that something "is" or "was" something, when actually, that isn't possible. Another point is that an author will use both similies and metaphors to create imagery, that is, pictures in the readers mind. so there you go, lecture over.
Using the things I have writen above, you can se that "His expression was gray" is a metaphor, because he could not actually go gray, or have gray expression.
"His heart sank like a stone" however, is a simile, since it compares his heart to a stone.
Hope this helps and good luck!