I assume you mean the softer white or natural tupperware such as CrystalWave (now replaced by Rock 'N Serve and Stuffables). It's not a good idea to microwave anything with tomato and grease in this type of dish, as the stain cannot be removed. If you only stored spaghetti with sauce in it, maybe it will come out with time. You can also rub in a paste of baking soda, let it sit a little, then rinse with white vinegar. Sometimes soaking in automatic dish detergent will do it. Putting it out in the sun for a couple of days might help too.
Plain tomato stains somewhat; you can usually avoid staining by rinsing the Tupperware in cold water before storing food in it, or spray with cooking spray. The greasier it is, the more likely it is to stain. The beta carotene in the tomato (and to some degree in carrots as well) absorbs into the plastic. When you add heat, it causes staining.
Heat tomato based products by microwave in Rock 'N Serve, or in glass or ceramic dishes.
Make a paste of cream of tartar and water....paste consistency. Take an old toothbrush and scrub it into the stains and then let it sit for a few hours before rinsing.