Boiling water. A phase change occurs when water boils.
The three common phases are solid, liquid, and gas. A phase change happens when one phase changes into different phase without a change in chemical composition.
When water boils, the liquid phase becomes a gas (water vapour or steam). The water does not change its chemical composition be each phase consists of water. Thus, boiling water involves a phase change.
Mixing paint does not involve a phase change because it stays a liquid during all stages of the mixing.
A suspension consists of two phases, but the phases do not change into other phases.