if it's a scratch where it's the other car's paint on your bumper, a clay bar should do it. If it's a scuff where you scratched off your clearcoat/paint, then a clay bar won't help- you'd need touch up paint and some good wax, if not a professional re-paint and clear coat.
How important is your bumper's condition to you, and how long do you think it will stay scratchless? How much will repairing it cost if indeed you need to touch it up or repaint it? Is it worth the trouble/money? --These are some questions you may want to ask yourself. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>
Good luck!