If the "Contiguous" button is unchecked, then clicking anywhere on my dog's black outline will automatically select every pixel in the joint that's black. (This will work with any color in a drawing, but black pixels are the ones we're after on this layer this time.)
Next I hit INVERSE in the pulldown SELECT menu.

That automatically selects every pixel on that layer that isn't black.

Artwork and the text for this demo are ©2001 by Howard Cruse / All rights reserved