**Special vehicles** are those that perform construction or special services on the roads. These vehicles must use **yellow lights** while working on the improvement or maintenance of the road. This is important because yellow lights serve to **warn other drivers** of the presence of these vehicles on the road, thus ensuring everyone's safety.

- Yellow lights must be on **only when the vehicle is working**. This means they should not be used in other situations, such as when the vehicle is not performing specific tasks on the road.

- Additionally, when these vehicles work on **motorways or highways**, they must turn on the yellow light from the moment they enter until they reach their destination. This helps maintain visibility and safety on high-speed roads.

Therefore, the correct answer is that special vehicles can use work lights, but **only when they are working**.