The OP asks why the white "landing light" is more readily visible during the day than the red or green nav lights.
The primary reason for the white light being more recognizable in this instance is that it is providing more photons than the green or red lights. The landing or taxi lights consume more power and produce more light than the nav lights.
In general, the eye sensitivity is a function of the photons received, adjusted for the sensitivity of the eye for the color(s) received, and adjusted for contrast (brightness gradient). The peak photoresponsivity of the human eye is about 560 nm, which is a green-blue color and shifts to 507 nm at night, which is more blue. The white light of the landing light is comprised of most or all colors which the eye is sensitive to, normally considered colors with wavelengths ranging from 400nm (violet) to 700nm (red), where nm is the wavelength of the light, nanometers, not nautical miles.
But to be clear, the visibility is a function of amount of radiation emitted, adjusted for the sensitivity of the eye, and in the OP case, the answer is simply that the landing lights have a much higher level of emissions than the nav lights.
As a tangentially relevant aside, the human eye, when night accommodated, has a very high quantum efficiency. Quantum efficiency is the ability to convert a photon (the smallest unit or packet of light) into a neural response which is recognized by the brain. Once impressed by the ability to observe lit cigarettes from 5000 ft and higher altitudes while doing surveillance work, I later investigated the responsiveness of the human eye. In general, a younger eye can reliably detect single photons. Things like available nutrients affect that, those nutrients including oxygen. Lay on the ground on a moonless night, and look at the stars, and then take a breath or two of oxygen, and suddenly there will be many more stars.
To summarize, the reason the white light cited by the OP is more visible is that it is giving off more photons which are detectable to the observer.