Colour is a physical property then how could be change in colour be a chemical change?

The colour change of a substance is not always an indicator of a chemical change. As changing the colour of a metal does not change its physical properties. However, in a chemical reaction, a colour change is usually an indicator that a reaction is occurring.

When two or more substances combine, they create one or more new substances, which sometimes have different molecular structures from the original substances, meaning they absorb and radiate light in different ways, leading to a change in the colour of the original reactants.

For example, during the rusting of metal, we observe that the metal has changed colour and turned orange. This change in colour is evidence of a chemical change.

[Note:  A colour change can be a chemical change only if the change is permanent, if the colour change is not permanent, then it is likely to be a physical change.]