Is a change in color when combining two substances normally a sign of a chemical change or a physical change?

Change in colour is a chemical change.

Explanation:

Because when two substances combine chemically, they create (a) new substance(s) that have/has different molecular structures from the parent substances, thus mostly likely absorb and reflect light in different ways.

pure substance: iron,copper

impure substance: water,charcoal

because natural gas is colorless, odorless, and tasteless, distributors add mercaptan (a chemical that smells like sulfur) to give natural gas a distinct unpleasant odor (it smells like rotten eggs). they locate the types of rock that are likely to contain natural gas deposits.