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.

https://photographingspace.com/ap-color/

Short answer: the colors they mapped to green is actually closer to red and the color they mapped to blue is closer to green, so it would have less cyan (blue and green) and more magenta (blue and red). It would probably look a little more purple-ish.