As Marcus outlined in his post you have to take advantage of the fact there is no purple anywhere else in your picture (or very little) so you can target that color.
Here's a file
RAR file here with a stack in PS based on your original picture.
I isolated the two layers of interest, one for the neb and on for the stars. Have a look at the settings. You'll get the gist of things.
In the end it all comes down to proper star masks and how much time you want to spend on it. Mask are easily done but using a difference blend between layers then refine the selection by contracting/expanding then feathering.
Also keep in mind the shadow highlight tool on stars only. It's a flexible way of deringing your stars while controling the cores and colors.
Finally blending layers on top of each other as "color", "lighter color", "Luminosity" helps as well getting rid of hard edges.