Forget ‘brightness’ it’s not relevant.
The suns ‘real’ kelvin temperature is somewhere around 5,800 degrees kelvin, but because the sun is not a perfect ‘black body’ (the standard for calculating kelvin temperatures of light) its adjusted kelvin temperature is somewhere around 6,400 degrees kelvin.
6,400 degrees kelvin is predominantly ‘bluish-white’ light and this is the best kind of light and kelvin temperature for vegetative growth – primarily because in nature the time of year Cannabis is in vegetative growth is during the long hot days of summer where the suns day length is the longest and receives the most 6,400k light. The reasons it changes for flowering is again because of the predominance of the kind of light found from the sun when Cannabis enters it’s floral stage – usually at the start of Autumn or Fall when the days get shorter and the nights longer thus reducing down the day length significantly. Sunrise and Sunset account for almost 50% of the suns light during the short days of Autumn/Fall and sunrise and sunset are made up of almost entirely red spectrum light due to the angle of the suns rays hitting the earths surface. Therefore as sunrise and sunset account for almost 50% of the sun total light output at that time of year it necessarily follows that 50% of that light will be entirely in the red end of the visible spectrum – hence the 2,700k color temperature being favored for flowering. In reality a mixture of red and blue light is required in flowering due to the other 50% of the spectrum being more blue in color.