Change the color of an Image

How can I take all the color out of a picture, except gray tones, and then add an overlay of another color (say green) while still being able to see the image?

Connie,
This is just one of many ways to do that:
Choose Layer: New Adjustment Layer: Hue/Saturation... In the Hue/ Saturation dialog, click the Colorize button. Then Use the Hue slider to select a color you like and the Saturation slider to make the color more intense. You can also adjust the lightness.