Twist on this, we had a white carpet that was stained so bad that 
steam/bleach cleaning wouldn't work. We bought liquid navy blue Rit dye and 
added one bottle to a gallon of water with a spray nozzle. We sprayed the 
carpet evenly in lines. We then let it air dry (with fans) for about 48 
hours. We then steam cleaned it using our Bissell and wow - looks like new 
carpet! Everyone thinks we had our carpet replaced. We used about three 
bottles of dye and water for the bedroom. We were so pleased that we have now 
purchased the "Rose" color to do another of our bedrooms. We used a ruler and 
put masking tape all across the doorway for a straight line. We were afraid 
it would bleed over but it didn't. We were scared to try it at first but our 
carpet was SO bad that we knew we couldn't make it look worse. I am so glad 
we did and will do all rooms as it becomes necessary! (HEY WHO PICKED 
WHITE/LIGHT CARPET ANYWAY!!)