Step One
Open your image.

Step two
click on the layers tab and click New Raster Layer.

Step three
name the layer what ever you want it doesnt matter

Step four
Now click the box image on the left toolbar

Step five
now make a box with the color you want

Step six
now double click the the layer, in the layer palette box. if you dont have a layer palette box right click on the color box and click on toolbars. then check in the layer palette box and click close.

Step seven
now change the opacity to what you like

Now your done! the finaly product: