Why does the Orion Nebula (M42) appear in shades of grey to an amateur observer?

The light is too faint to register with the eye's color-detecting cones.

The perception of color in astronomical objects viewed through an eyepiece is dictated by the limitations of the human eye, particularly under dim conditions. While professional imaging using instruments like the Hubble Space Telescope employs long exposures and specific filters to assign colors corresponding to spectral signatures (like the red glow of ionized hydrogen in emission nebulae), the light gathered by a personal telescope setup is often too faint. The eye's rods are excellent at detecting low light levels (scotopic vision) but are not sensitive to color, whereas the cones require significantly more light to activate their color-sensing capabilities. Consequently, under typical observing conditions for objects like the Orion Nebula (M42), the visual experience is rendered in monochromatic shades of grey.