If H2 fires are stoichiometric, I thought they were almost invisible. What is causing the orange color in the flames shown in numerous slides during the presentation?

Hydrogen flames can be nearly invisible in daylight, especially at low flowrates. The concentration of hydrogen does not have much effect on the color of the flame. Many hydrogen incidents or fires will have a bright orange hue, or even yellow flames. The color is primarily caused by contaminants that is either naturally in the air in certain environments, swept into the air during the release (such as duct), or surrounding materials which are also burning