The colour of fireworks When the electrons that are excited by the energy of burning drop back into lower energy levels they emit light at the wavelength of colour.

When metals are burnt, metal oxides(some coloured) are formed.

When metals are burnt under high temperature, will they turn to plasma state? If so, should the metal ions be responsible for the colours and sparks?

Which one is the reason for the beautiful colours of fireworks?
The colours of fireworks are caused by the electrons emitting energy when dropping orbital levels. Metal oxides are solids, mainly dull colours (even the yellows and reds are not in the same class as firework colours), and could not cause bright light in the air. The temperature of fireworks is far below that needed to form a plasma.