As long as the tomato stain is just from storing sauce in the plastic bowl, filling the bowl with water and a bit of bleach and letting it sit several hours may remove the stain.
If you made the mistake of heating spaghetti sauce in that bowl, then the stain will probably be there when you toss the bowl.
Regardless of what is said when this brand is advertised, I do not agree with using these bowls in the microwave or using an electric mixer in them, as heat will damage the inside of the bowl, and the beaters from a mixer will leave permanent scratches.