here is the original image (unud building).
then use the Rectangular Marque Tool to make a Rectangular Selection.
Use Shift + Ctrl + U or Image-->Adjusment-->Desaturate. Below The result.
This Desaturate Effect can be use as Past effect (grayscale side) or Present effect (color side)of the Picture. You can also use another way beside this desaturate effect..^_^