The last rays
Per definition visible light is an electromagnetic radiation in the approximate wavelength range from violet (~400nm) to red (700nm). Taking pictures, we have a whole bunch of tools in our hands to paint with light, which actually is the meaning of photography. Independent of definitions and technical and physical features, photography is all about capturing an atmosphere of a scenery and to provoke emotions.
A sunset, grazing light, and the last rays of the day coloring the sky in orange and red is the perfect atmosphere of describing what warmth in regards to light means to me. It’s a glimpse of coziness that spreads throughout my body when I watch the evening sun.