The green Nano 5mW and 15mW laser will be brighter than either the red or purple Nano lasers even though these have higher power.
This is because the color green is perceived better by the human eye compared to red or purple. Due to this, it is more visible and brighter. The beam will be visible at night even without any smoke or fog.
The purple Nano laser has the lowest visibility as it is closest to the UV wavelength.