Hi Lynn, I mix a small amount of bleach with water and cover the stain and let soak. Here's some additional methods.
Once you get it clean, the next time you store tomato based foods in your plastic containers spray with Pam baking spray before placing the food in the container.
To prevent it from happening next time, I wash the item with COLD soapy water then wash it in HOT soapy water. Works for me! Good luck
Hi Lynn, you can also rub the stained tupperware with lemon. This video will show you how - https://www.youtube.com/watch?v=i84vx4gFi38
If you can't get out the stain, Tupperware has a lifetime guarantee and they might replace it. They've replaced several pieces of mine that have cracked lids.
If the tomato sauce has been in meat sauce and then microwave in the Tupperware, this is going to be difficult.
The fat from the meat on the oil-based plastic Tupperware, chemically stains it.
Lots of tips here: