After you desaturate the entire image, you can't color it without either un-doing the desaturation, or creating another layer that you can add color to.
Exactly. You need to have colour there in the first place.
If you shot an image with a black and white setting in your camera, you cannot get that colour back into the image. You'd have to manually use a paint brush to do so, which would probably yield horrible results.
Which is why I never shoot BW in camera, I always convert to BW in processing.
If you have created the two layers, top desaturated and bottom coloured, and you are using the erase tool to bring out the colour from the photo underneath and make a mistake and take out too much, simply undo.
That is why the Layer Mask function in Photoshop (I dont think PSE has this) is a great tool, as you don't really erase something, you just chose to hide it and can always unhide it if need be.
I would highly suggest heading to YOUTUBE and do a search on a video that shows you how to do this.