Why should pyrotechnicians worry about the yield of the reactants in a firework? How does the yield affect the firework and the environment?

The colors emitted by fireworks are products of chemical reactions. These products can be harmful to the environment, that's why, in other countries, it is prohibited to use fireworks, unless during New Year or Christmas. For example, blue fireworks from copper release cancerous dioxins, green sparks come from barium which is radioactive, etc.