You can do it more than one way in PSP but one way is to duplicate the layer. Then on the top layer go to ADJUST, then HUE AND SATURATION, then HUE/SATURATION/LIGHTNESS and drag the hue button down to lowest setting (if that doesn't work change the saturation). Then after that using your select tool, draw around the part you want to colorize and either cut if you want the original color or colorize or paint it if you want a different colour.
OR close out the top layer after you use your select tool then right click and promote selection to a layer. Move that layer to the top and delete the bottom layer. Make sure you do selections/select none. Merge your top layer into your bottom layer by right clicking on the top layer on your layer palette, and click on merge/merge down.
Here is a picture to work with to practice color splashing.
Remember, you are going to turn part of it black and white and keep part of it colored using the directions above.