Have you ever had toothpaste drip on your shirt or sweatshirt and it actually left a bleached stain? Unfortunately, I did. I was brushing my teeth wearing a brand new sweatshirt and the toothpaste dripped a bit right down the middle and when I washed it, it now looks like I spilled bleach on it. I had no idea that was possible, but beware......it does.
Try a Carbona Stain Devil. There are 9 different varieties of them (and you can find them in the laundry soap aisle at Walmart or the grocery store. They are in tiny yellow bottles and are usually less than $2. They are AMAZING! They even work for set in stains that have been washed many times before. I LOVE THEM! =0) For toothpaste, it says to use the "Makeup, Dirt & Grass" Stain Devil. Here is a PDF from their website that breaks down every stain imagineable and which Carbona Stain Devil to buy in order to get rid of it......
http://www.carbona.com/sd_new_chart.pdf
Good luck! =0)
If it was a whitening toothpaste you probably can't do anything to fix it. Unless you want to mess with trying to dye it. Those toothpastes have a bleaching ingrediant in them. A lot of acne products will do the same thing. We have several towels that have "bleach" marks on them. I never use bleach so I couldn't figure it out. Then I saw something in the Hints from Heloise column in our newspaper about it.