What color should I use on my hair

The shape of your face does not have anything to do with color, but the haircut/style. You need a color analysis. What I would need is to see your skin color. Depending on your skin tone (yellow, olive, pink, neutral, or even bluish hues) and the depth, light, medium, dark, along with what haircolor you choose, you could actually end up looking much older, washed out, and actually enhance wrinkles, age spots, and dark circles under the eyes. Without this I would say go about 1 shade lighter or darker than what your natural haircolor was when you were in your 20's. If your hair was really dark, go a little lighter, if your hair was blonde, then go a shade darker, if your hair was light to medium brown you can go one way or the other. Either way, the change will seem very drastic and may take some time getting used to.
Based upon what you said about what you have, I would suggest finding a brown close to the color you have in your hair (choose a neutral based color, no ash, gold, red or violet bases). You can go to Sally's beauty supply and purchase Clariols Grey Busters choosing a color that has just an "N" in the color number (ie. 83N). Then have both highlights and low lights added to give you dimension and keep if from causing a washout. I would do medium or dark blonde highlights and chestnut low lights (again staying away from the reds, ashes, and yellow based colors as this could look very unnatural, unless you have had red highlights in your natural hair color at one time).
Good luck, Susu