Most reflective color

It is hard to destinguish what is the brightest color becaus the color itself will mess up the perception of the brightness. Even if green light at approx 530nm is the wavelength that the eye is most sensitive to, I would say that yellow or a color between yellow and green are the brightest seen in nature.

If we make an image on the computer of a 100% green and yellow surface and convert both of them to grayscale image, you will see that there is a no difference in brightness. However, the yellow color on the screen appears the brightest of the two because the computer screen add red light to the green light to make yellow. Adding another light to the green light, will make it brighter.

In nature there is no colors that is monocromatic - which means that the color that is reflected by the object does not consist of only one wavelegth. The nature reflects more than that color so the nature is more pastel than monocromatic. Possibly the reason why yellow colors in nature appears brighter than green. However, some poisonous animals, as some frogs, do have a very vivid green color that might appear brighter than any other color in nature.

Flourecent colors is another matter. These colors consist of a material that emit visible light when blue, violet and ultraviolet light shine on them. In late evenings, the deep blue sky will make such colors shine brighter than any other colors. Not only in the late evening but also at daytime ofcourse because there is not only the visible light spectrum that makes them visible, but also the blue, violet and ultra violet spectrum.

Most materials are fluorecnt, but many of them convert visible light into infrared light - which we cannot see. However, if you shine a bright blue light on a typically non fluorecnt surface of yellow, you get almost white light in return. If you do the same to a red surface, you get yellow and amber color in return.
An object is white when it reflects more or less any color – any frequency – and it reflects it in a random direction: the light gets scattered.

For this reason, the photons arriving from a specific point of the white object (e.g. a point on the paper) are photons that arrived there from random directions in the space and they have random colors. Because the white object mixes the directions, it mixes the colors, too, and the mixture of all colors (including red, green, blue) looks like white light.

A mirror is an object that reflects the photons in a specific direction – the direction that only changes the sign of the angle between the photon and the (tangent to the) surface of the object.

An ideal mirror would reflect all colors perfectly so it would appear colorless. However, there's some scattering from metals such as silver, platinum, and iron. This is mostly independent of the frequency so the metals appear color-neutral i.e. grey.

Copper, gold, and others tend to scatter especially longer [thanks for the fix] (redder) wavelengths, so they appear redder – orange or yellow (shiny yellow i.e. golden) etc. Incidentally, individual gold atoms not bound in the macroscopic body would appear greenish, not yellow.