There are many ways to do what you want, depending on the specific type of
results you need. You could create a rectangle over it and change the
opacity of the rect. You could apply live effects or filter to your image,
such as Adjust Color > Hue/Saturation and check the colorize box. If you
need fancier results, there are always brush techniques, etc. If you want
to go monochrone, I would do the Colorize option.