Bruises are like Polaroid photos... they often take a little while to develop. Don't panic, eyebrows often bruise. The piercing was a major punch through the tissues and being on the surface of the face, the internal bleeding is showing. Yellow isn't necessarily bad, often a slightly older bruise will have this colour. If the brow is shrinking and does not have fresh tenderness or oozing, chances are the brow is ok.