It's harder for people to remove your signature/watermark if you put it on a gradient area instead on an area with solid color, and if art theft gets really bad or you really don't want someone to steal your art piece, don't be afraid to put watermarks ALL OVER the art piece.
Quick comparison drawing of BadBoyHalo and GoodBoyHalo, nothing much is different between them. Most noticable difference is the inverted colors, because that's the point >_<