This image consists of 5+ hours data (100 min luminance and 75 min red, green blue each).
The blue color comes from the young, hot star in the middle (SAO 19158) - it has 10 times the mass of our sun. Zooming into the center of the nebula shows some reddish glow:
This is the result of ultraviolet light (from the center star) converted to visible light.
I found this great sketch on the web that illustrates how we see this nebula:
Title: Structure and physical properties of the bipolar outflow source NGC 7023
Authors: Watt, G. D., Burton W. B., Choe, S.-U., & Liszt, H. S.
Journal: Astronomy and Astrophysics (ISSN 0004-6361), vol. 163, no. 1-2, July 1986, p. 194-203
Bibliographic Code: 1986A&A... 163.. 194W