i think in human sight the color ff0000 may has a medium brightness between 00ff00 and 0000ff

You're correct that the color represented by `ff0000` in hexadecimal code—red—is generally perceived as having a medium brightness compared to `00ff00` (green) and `0000ff` (blue) under typical viewing conditions. Here's why: 1. **Wavelength and Sensitivities**: Red light is at the longer wavelength end of the visible spectrum, which still falls within a range where human eyes are sensitive, albeit not as sensitive as green light. 2. **Cone Cells Response**: The red-sensitive cone cells (L-cones) have a response curve distinct from green- and blue-sensitive cones. Their density and sensitivity levels create a perception of moderate brightness for red light. 3. **Brightness Perception**: Different colors appear to have different levels of brightness even if the actual intensity is the same. This is due in part to how our brain processes different wavelengths. 4. **Contrast**: Red contrasts strongly with many other colors, which can give it a sense of prominence and "medium" brightness compared to cooler greens and blues. 5. **Adaptation to Light Conditions**: The brightness of red can seem balanced because we see a good deal of red-spectrum light from various light sources during daytime, evening, and many artificial lights. 6. **Hue Influence**: The psychological influence of color on perception cannot be discounted. While red itself could be processed by humans as a color carrying importance or urgency, this doesn’t diminishing its perceived brightness but rather influences its emotional response. 7. **Environment Exposure**: Our environment presents us with various shades often, and red is common enough - whether in natural or manufactured settings - to not stand out as unusually bright or dim. Thus, considering these factors, red (`ff0000`) often appears with an intermediate brightness between green (`00ff00`) and blue (`0000ff`), creating a visual balance. However, keep in mind that perceived brightness can greatly vary among individuals and their specific conditions and backgrounds.