I've seen the type Dave mentions probably a dozen times, cylindrical casters on bed frames. Stain is almost a reddish pink...never got them out.
I've found a solution.
I came here after googling exactly this problem. I'm in the process of moving out of our apartment, and I noticed that the casters on our bed frame left a ton of these reddish pink marks all over our berber carpet. I too tried a number of things to get rid of the stains. I tried three or four commercial stain cleaning products without any luck. We had our carpet professionally cleaned, removing all the other stains in the apartment but these pink stains remained unchanged. I did have some luck when I tried making a paste with baking soda and hydrogen peroxide and massaging that into the carpet. I would then clean the area with water and reapply. This lightened the stain to some degree but it was still very noticeable. I tried using a 50/50 mix of water and white vinegar, again with very limited success. I tried checking the website Larry Cobb mentioned but I had no idea to had to find the product he was referring too. I was about to e-mail the company to find out more expecting to pay even more to try out another product on a half dozen small pink stains. I was coming the conclusion I'd be stuck with these stains and was worried I'd end up having to pay to have the carpet replaced.
Then out of complete desperation I tried something mentioned several other times for stain removal. I soaked the area with a mixture 2/3 vinegar with 1/3 water. I took a few sheets of paper towel and placed them over the stain. I then used an iron at a medium heat setting. I applied the iron for several seconds at a time, making sure not to leave it in place long enough to burn the paper towels or more importantly the carpet. I would do this until the mixture had more or less all been absorbed into the paper towel. SUCCESS!!! The results were apparent immediately. Each stain required around 5 or 6 rounds of this technique, but the results is a completely stain free carpet. The pink/reddish stain is completely gone! It did make the room smell like vinegar for a little while, but it was worth it for this result.
If anyone else is looking for a solution to these horrible bed roller stains this was not only incredibly cheap but also better than any of the products we purchased. I picked up a gallon jug of the vinegar at the grocery store for around $2, the store brand is fine. The only other cost would be an iron (if you don't have one) and paper towels. A towel might work just as well, I'm not sure. FWIW I've always used vinegar to help with cleaning, it's especially good at cleaning shower heads if you have hard water and mineral deposits. Put some in a spray bottle and try it out around the house or to give your shower a daily mist. It's versatile, effective, cheap, safe and natural.