B&Q lawn feed and lots of tender loving care
Or failing that if you have Elements or equivalent you just create a new layer and then choose the paint brush tool at whatever size you feel comfortable with. Then choose a darkish green and paint over the grass area.
You will see that you just get a block green colour that covers over the grass but don't worry!! Once the area is painted over with green then simply use the opacity slider on the layer side window and reduce the percentage to about 30% or until your grass reappears and looks how you like it.
Once done just merge/flatten the layers as normal.
You can adjust your paint brush opacity to achieve the same effect but I find it simpler to do it the whole layer.
Also works with sky's, hair colour, eyes or whatever you want to colour in a B&W image.
Have a play and feel free to ask if I have not explained clear enough and I will try and walk you through any details you are not sure on.