If this has happened because of spices, that’s tricky as the colouring gets absorbed into the plastic (and the cheaper the plastic, the harder it is to remove). But here are a few things you could try.
1. Wipe with paper towel dipped in cooking oil, then wash in hot soapy water.
2. Wipe over with lemon juice, which is a natural bleaching agent. 3. Soak overnight in a solution of Milton (15ml per 2.5l water).