To comprehend the reason for this phenomenon it must be appreciated that the human eye functions best when the complex chemical combination in the vitreous humour is in total equilibrium.
The four principle chemicals in this process (known collectively as Retinaldehyde) are Dimethyl, Trimethyl, Cyclohexane and Nonatetraenal.
The Dimethyl feeds and maintains the sensitivity of the retina, the Trimethyl nourishes the rods, the Cyclohexane the cones, while the Nonatetraenel enhances the conductivity of the optic nerve. Curiously, all four chemicals are degraded by the same chemical toxins but to varying degrees.
Thus nicotine, alcohol, caffeine and chocolate all react mostly with the retina but also to a lesser but still substantial degree with the rods, cones and optic nerve.
Consequently, Percival Lowell, who observed Mars at the Flagstaff Observatory puffing on Cuban cigars and swigging from his bottle of medicinal brandy to counteract the chill of the Arizona night air, spent many happy nights drawing the pattern of blood vessels snaking across his own retina and mistaking them for the "canals" of Mars and not realising he had actually invented the retinal scan.
Edward Emerson Barnard was the first skilled observer to figure out just what Lowell was seeing by, like Lowell, stopping down his own 26" refractor to 4" and observing alternately with his right eye and then his left eye, noting that on nights of indifferent seeing when the canals were most evident, their apparent structure varied depending on which eye was used.
Barnard however, being a self made astronomer and lacking a formal University eduction in all the fashionable superstitions that the astronomy establishment insisted all budding astronomers must know, had been savaged for suggesting he saw evidence for dark nebulae at such a time when conventional wisdom said they did not exist.
Accordingly, rather than publish material which could make Lowell look like the eccentric old goat he really was, Barnard stayed quiet and kept his job.
A look at the mechanics of both colour and black and white vision reveals the following:
The Kruithof Curve - In colour vision, the colour experience of a given light mixture may vary with absolute luminosity, because both rods and cones are active at once in the eye, with each having different colour curves, and rods taking over gradually from cones as the brightness of the scene is reduced.
This means, for example, that light with a colour temperature of 6000K may appear white under high luminance, but appear bluish under low luminance.
Under the same low luminance conditions, the colour temperature may need to be adjusted to, say, 4700K, to appear white.
This effect leads to a change in colour rendition with absolute illumination levels that can be summarised in the Kruithof curve, named after A. A. Kruithof – Source: Wikipedia.
Strictly speaking then, NGC 3918 should appear bluish if it is highly luminous and white only if it is of low luminosity. The fact that it can at the same time appear blue to direct vision yet white to averted vision calls for an in-depth study.
As the brightness of the scene decreases the brightness of red colours decreases more rapidly than those of blue colours, this is the so-called Purkinje effect.
Let us consider Beta Crucis - A bright blue star with an 8th magnitude, ruby red companion. The question here is whether the brighter Beta Crucis appears so brilliant only because of its blue colour rather than its position on the Headstrong Rustle diagram.
The Purkinje Effect - A close look at the Purkinje effect may cast new light on this issue. According to Wikipedia, the Purkinje Effect (sometimes called the Purkinje shift or dark adaptation) is the tendency for the peak sensitivity of the human eye to shift toward the blue end of the colour spectrum at low illumination levels.
This effect introduces a difference in colour contrast under different levels of illumination. For instance, in bright sunlight, geranium flowers appear bright red against the dull green of their leaves, but in the same scene viewed at dusk, the contrast is reversed, with the petals appearing a dull red against paler green leaves.
In visual astronomy, the Purkinje shift can affect visual estimates of variable stars when using comparison stars of different colours, especially if one of the stars is red. For example, the companion star to Antares is strictly yellow but looks green to the human eye when observed through a telescope.
The effect occurs because the colour-sensitive cones in the retina are most sensitive to yellow light, whereas the rods, which are more light-sensitive (and thus more important at dusk) but which do not distinguish colours, respond best to green-blue light. This is why we become virtually colour-blind under low levels of illumination, for instance Moonlight.
The Purkinje effect occurs at the transition between primary use of the photopic (cone-based) and scotopic (rod-based) systems: as intensity dims, the rods take over, and before colour disappears completely, it shifts towards the rods' top sensitivity.
Incidentally, the same lack of sensitivity to red is why red lights are used in instrumentation panels: the red light does not disrupt the night vision (because the rods do not respond to it), and the fovea is largely L (red-sensitive) cones, hence the instrument panels are easy to read.
Though commonly used in amateur astronomy for semi-religious reasons, red lights are not worth a damn to read by, but are more suitable in the form of harmless red LED's on instruments or computers.
In fact the US military in exhaustive tests designed to establish which form of lighting can enable soldiers, sailors or airmen to read charts or documents without destroying their ability to function in low light situations or at night established that any monochromatic light source of similar luminosity is far better than red light, with dim green light performing particularly well.
Getting amateurs to swap their red lights for green however is a big ask, with habit, semi-religious beliefs and the white light Gestapo at star parties combining to preserve the status quo.