The first thing we need to do is select the area of the image that will have its colour changed. The best way to do this under the circumstances is to use the Colour Range selection tool. This is a menu option that allows you to select an area of an image that contains a particular colour. You'll find in the Select menu.
When the Colour Range dialogue window appears, the default setting is to select a sampled colour, and the cursor will have changed to an eye-dropper. Other colour range selection options include pre-set colours as well as highlights or shadows.
Click with the eye-dropper on a middle tone area of the colour you wish to select, and you'll see the black-and-white thumbnail in the dialogue window change. The white areas represent the area that will be selected. You can change the tolerance of the selection by moving the "Fuzziness" slider. In this case I want to be sure to select all the green areas, including the slight reflection on the white background, so I'll move the slider almost all the way to the right.
We don't want the stalk to be changed, so we're going to edit the selection slightly to be sure to exclude the stalk. We can do this using another very useful Photoshop filter, the Quick Mask mode. This is activated by clicking on a button at the bottom of the tool palette.
In Quick Mask mode you'll see that all the unselected areas of the image now have a red tint over them. This is the mask, and in this mode it can be altered by painting using any of the normal Photoshop painting tools. You'll also notice that the background/foreground colour pickers (located just above the Quick Mask button) have changed to black and white. Painting in black adds to the masked area, removing anything that you paint over from the selection. Painting in white deletes areas of the mask, adding to the selection. Using a small hard-edged brush I paint over the stalk in black, covering it with the mask and removing it from the selected area.