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.