Zoom in on the apples in the photo so they fill your workspace. Activate the Paintbrush tool, pick an appropriately-sized round brush, and set opacity to 100%. Set the foreground color to black (you can do this by pressing D). Now click on the layer mask thumbnail in the layers palette and then begin painting over the apples in the photo. This is a good time to use a graphics tablet if you have one.
As you paint, use the bracket keys to increase or decrease the size of your brush.
[ makes the brush smaller
] makes the brush larger
Shift + [ makes the brush softer
Shift + ] makes the brush harder
Be careful, but don't panic if you go outside the lines. I'll show you how to clean that up next.
Optional method: If you're more comfortable making selections than painting in the color, feel free to use a selection to isolate the object you want to color. Click the eye in the layers palette to turn off the grayscale layer, make your selection, then turn the grayscale layer back on, click the layer mask thumbnail, and then go to Edit > Fill with FG Color, with Black as the foreground color.
