2. Press D on your keyboard to set the foreground colour to black
3. Select Layer > New Adjustment Layer > Gradient Map > O.K. > double click directly on the gradient map > this opens the gradient editor
4. Click somewhere under the black to white gradient gradient => this will add a colour stop
5. Click directly on that stop itself and choose any colour you want from the colour picker. Keep clicking or moving the vertical hue slider
6. Click O.K. 3x
7. You can now alter the opacity of this layer to your liking!
Also try:
Instead of adding one colour stop as in step 4 you can add 2/3/4... and click each time on the stops to choose a colour => this way you can "rainbow" colour your image...