Why is red light seen as "warm", although it is still lower in energy than blue?
Red and blue light differ in energy density, they are located on opposite sides of the rainbow. Blue light oscillates almost twice as fast as red and has a correspondingly shorter wavelength. And because it swings faster, the particles of blue light are also higher in energy. Nevertheless, red light appears warmer.
This is the difference between physics and psychology. How a color affects us depends not just on whether or not the corresponding rays are energetic, but it depends on how the brain processes signals. And especially when it comes to colors, very archaic associations play a role.
Red, yellow and orange - these are the colors of fire, of embers. The sun appears red in the evening sky. And red is also the color of blood. Blood means heat: If we are hot and our fingers are warm, they are red. This already happened to our early ancestors, and that's why this association of red = hot is probably anchored deep in our perceptual apparatus. Conversely, if we consider what is blue: blue is, for example, water - and water is cool.
Now you can take this even further and say: Red light is not only psychologically warm, but also physically warm - if we think for example of infrared lamps, they really warm us. Which is why they are used in medicine, while, conversely, of course, no doctor would think to treat a person with UV lamps. Because UV light is aggressive, it can damage the skin and cause cancer.
This is because UV light is even higher in energy than blue light - the high energy density makes the light so aggressive. Conversely, infrared light, while physically even lower in energy than red light, is seen as heat radiation.
This has the following background: Basically, every body has a natural radiation, and this characteristic radiation depends only on the temperature. Therefore, one can, even if it is pitch dark, distinguish hot and cold items with an infrared camera, because warm objects glow stronger in the infrared spectrum. There is a clear relation between the temperature of a body and the kind of radiation that it emits.
If something has room temperature, then we do not see this radiation, it lies in the infrared and our eyes can not see infrared radiation. But when things get warmer, the radiation changes and shifts into the range of visible light - in other words, it shifts from infrared to red. That is why red-hot objects are red.
At still higher temperatures, the heat color would in fact shift to blue, but when a body glows blue, it would be even hotter than the sun, and in everyday life, we almost never have to do with such extremely hot objects. That is why red is the color of a warm, glowing body.