This is what you'd do in Photoshop (I'm the opposite to you - PSP just seems clunky to me and I can't bear using it) so you'll have to translate it.

Select menu>colour range.
Click the colour in the image you want, then adjust the slider to get the range of shades you want.

Using that as a starting point, go over and retouch any edges that need it. This sort of operation isn't something you can just do with a couple of clicks, unfortunately.

If you want an easy option, select the edge around it, exapnd the selection by a few pixels, feather it the same number of pixels and you'll have a neat looking edge, with a blue glow effect.