Go to the black-and-white image and select it all by choosing Ctrl, A. Copy it by pressing Ctrl, C or Edit, Copy. Go to the colour image, making sure the new ‘B&W' layer is selected, then hit Ctrl, V or Edit, Paste. The black-and-white image will be pasted on to that layer, covering up the colour background image.
Click the anchor button in the bottom of the Layers window and close the black-and-white image window. In the Layers dialog box right-click the B&W layer and select Add Layer Mask. In the Add Layer Mask options dialog that appears select ‘White (full opacity)' and hit ok.
Now we'll ‘paint' transparency into the black-and-white layer. Make sure that black is selected as the foreground colour in the toolbox and that you've clicked on the layer mask part of the B&W layer (the white square to the right of the image thumbnail) to activate it. Choose Dialogues, Brushes and select a fairly large brush.