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It depends what you mean by correct colour. It also depends on the nebula.
If the nebula is a reflection nebula then it will be the colour of the stars that are illuminating it. If it is an emission nebula it will be the colour of the particular emission lines that it is emitting.
If it is a planetary nebula it is mostly an emission nebula so the same thing applies.
How you interpret the particular frequency of light being emitted is up to you and your eyes. Many people try to balance the picture so that the colour balance is similar to our sun. This results in the red images you see of the Ha nebulas.
Many digital CCD cameras are blocked in the deep red and infrared. This makes the light of Hydrogen alpha (which is a deep red) poorly recorded by the sensor. This is why the red colour of a Ha emitting nebula (like the orion neb) is only poorly recorded. If you use a CCD that can see into the IR then the Ha is recorded much better. The orion neb has both reflection and emission components hence the variation in images that you see.
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