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.

answer: the rate of diffusion increases with increase in temperature.

increase in temperature, increases the molecular/particles speed (kinetic energy. diffusion means movement of particles through the material.

generally, the heavier the particle the more it interacts with the environment and more the temperature matters. so higher temperature results in higher diffusion.