Why isn't our local interstellar space colorised?
This is really more of a biology question, I think, so: The Mark-I Eyeball is far less sensitive to color than to intensity.  There are rods and cones in the retina. The neural signals from the rods are used to report intensity only. There are different spectral sensitivity curves for different cones (typically three types in humans), so the relative outputs from all cones are used to generate color information.  However, cones are less sensitive than rods, so for dim light such as that reaching us from nebulas, only the rods produce a neural output.