If you want a quick way to find the RGB value for a color on your screen, MouseZoom is will give you an exact value for any pixel you point at.
This tiny 51KB program doesn't need installation, so you can keep it on a portable drive if you want. It has a small window, with a zoomed view of wherever your mouse is pointing. The zoom can be set anywhere from 2x to 50x if you're dealing with super high resolution images. When you've found the point you need, clicking will freeze MouseZoom, so you can note the RGB value. You can also see hexadecimal color values if you choose.
This is a fairly old program, and while it functions perfectly, it's not really attractive. This is countered by its tiny size. It won't bother your processor a bit. It's a shame you can't copy/paste the RGB or hexadecimal values to the clipboard, but otherwise it's difficult to see what it is missing.
For anyone who need to color match, MouseZoom is a really quick and easy way to get exactly the right shade.