There are several parts to the answer of your question. First of all, emission nebulas are not very bright, so no telescope can give a picture as bright as a l

There are several parts to the answer of your question. First of all, emission nebulas are not very bright, so no telescope can give a picture as bright as a long-time exposure does. If you can see a nebula through a telescope at all, it will be very faint. Which triggers another effect in your eye: the cells for color reception are not very sensitive. Like with general night vision, you will see nebulas usually only with your light-sensitive receptors, which don't see colors. So it will appear in a grey-greenish color. High quality pictures of nebulas are taken at very specific wavelengths, of common emission frequencies, you listed them. Even at high brightness, they wouldn't directly convert into a good color picture, as 500nm is turquise, 656 and 672nm are very deep red. A color image converting these wavelengths directly into RGB-values would be not very impressive, it would look more like the bottom image on the page. So usually a color mapping is used to generate impressive images which also show a lot of the detail information. With 3 different "colors" in the source image, you can apply an arbitrary transformation to generate an RGB-image. For example, most images from the Hubble telescope use a common mapping which is consequently called the Hubble-telescope mapping. Like shown on the page, you can create very different looking images from the same data set by choosing the color mapping.