Fill a medium pot with water, and bring to a boil. Add corn; cook until kernels are tender, about 4 minutes. Using tongs, transfer corn to a platter; let cool. Discard all but 3 cups cooking water. Use a sharp knife to shave kernels from cobs (to yield about 3 cups); set aside.
Add vinegar, sugar, turmeric, mustard powder, mustard seed, salt, and bay leaf to pot. Bring to a simmer over medium-high heat. Add cabbage, red peppers, onion, and corn kernels; stir to combine. Bring to a boil; immediately remove from heat, and let stand 5 minutes.
Strain mixture, and let liquid and vegetables cool in separate bowls; discard bay leaf. When cool, strain liquid through a fine sieve into bowl with vegetables.
Serve relish chilled or at room temperature, using a slotted spoon to leave liquid behind. Store in an airtight container in the refrigerator.