Explanation:

- Dull: Lacking brightness, vividness, or excitement.

- Option 1 - Coloured: Refers to something that has color. Not directly opposite to "dull" as dull can also refer to lack of brightness or stimulation, not just color.

- Option 2 - Bright: Means full of light or vivid. Often used to describe something opposite of dull, as it implies liveliness and vividness.

- Option 3 - Clever: Describes someone intelligent or quick-witted. Not related to dull in terms of brightness or vividness.

- Option 4 - Youthful: Means having the qualities of youth. It doesn't directly oppose the concept of dullness.

Bright is the most appropriate antonym for dull in the context of light or stimulation.