On a dull day, with dreary light grey skies, you can consider doing a few things to add interest.
- Make the background darker by using -1 to -2 stops of exposure compensation
- Get close to a subject that can add interest
- If possible, make that a colourful subject
- Use your flash!
- Set flash compensation appropriately as needed
You will see that even on a dull day you can make interesting images.