點解burn Mg有brilliant flame?

During combustion of magnesium, magnesium reacts with oxygen in air to form
white powder magnesium oxide:

2Mg + O2 = 2MgO

This powder contributes to the brilliant white colour of flame.

As for other metals, their combustions in the presence of oxygen form oxides of other colours

e.g. Potassium - potassium superoxide KO2 is yellow

BUT THEN the MOST important reason to explain why some other metals such as K, Na, Ca do not have brilliant white colour lies in the characteristic flame colours due to electron transition

Upon combustion in non-luminous flame, the heat energy excites valence electrons of these metals to higher electron orbitals. When these excited electrons return from excited state back to ground state, the released energy E=hf is in the form of light of characteristic wavelength.

As for Al, Zn, Fe, Pb, this is nature - they simply do not release absorbed quanta of energy with characteristic wavelength.