Observing the brightest deep sky objects through a large aperture telescope can just begin to show the colors of a few of these, although most of the time one can just distinguish between bright reflection nebulae and emission nebula only when seen adjacently. Since the emission nebulae have a prominent emission line near the peak of the eye's response in the green this is how brighter emission nebulae appear. Usually reflection nebula appear a more neutral gray. A few objects such as the Orion nebula are bright enough to just begin to stimulate color vision in a large telescope. Here is my impression of the colors visible in a 24 inch telescope, processed from a painting inspired by a Malin photograph.
More detail is shown than can be visually distinguished.
Most people can distinguish the green of the 'core' regions, but the other hues are faint and only visible through large telescopes. I have thought they were just detectable through a 10 inch, but this is qualified by a quirk of the human eye, color afterimages of the bright green areas 'washing over' adjacent darker areas with the complimentary 'red' color. There is also a mental image of what we are seeing based on photographs which can influence ones impression.