but if different color light are in equal physical brightness

Under the condition where different color lights have equal physical brightness (the same energy or power of light, which is often measured in watts), the perceived brightness can differ due to the varying sensitivity of the human eye's photoreceptors - the rods and cones - to different wavelengths of light.

1. Wavelength Sensitivity of Photoreceptors: The human eye contains three types of cone cells, each sensitive to different regions of the visible spectrum. Those sensitive to medium wavelengths respond the most strongly to green light. Cones sensitive to long wavelengths are less sensitive than those for green but more sensitive than those for short wavelengths. Cones for short wavelengths (blue light) are indeed less sensitive, making blue appear dimmer.

2. Brightness Luminosity Curve: This difference is shown by the luminance function of the visible spectrum, which peaks for green light at around 555 nanometers. This means that under equal physical intensity, green light will appear brighter to us than red or blue light, with blue light appearing the faintest when conditions are optimal.

3. Differences in Rods and Cones: In low-light situations, the effect is even more noticeable because rods, which are more sensitive to green-yellow light and dominate vision in dim conditions, have a peak sensitivity at about 498 nanometers. Therefore, at night or in poorly lit environments, green light appears brightest.

4. Colored Shadows: You can observe this phenomenon in everyday life as well. Often called "Purkinje effect" in colorful shadows under white light, you will observe that blue objects appear darker compared to green and red ones.

However, it's important to remember that "brightness" is a perception, and different people might perceive differences slightly differently due to individual variations in the eye's structure and color perception abilities.