Hi Scott,
After having a closer look at your image, I feel that there is a bit of field rotation that affects registration of the colours. When registering individual images into colour images, and then when combining them into the final image, from my experience, it is important to correct for field rotation, especially with longer sessions, even with an equitorial mount. Not sure how to do it in PhotoShop, but field rotation can easily be corrected for in Nebulosity, and PixInsight does it automatically when registering images.
Also, I will echo Allan's advice; in stretching images, when you move the black point, unless there is a particular reason for doing otherwise, always avoid clipping data- I recall someone somewhere wrote quite wisely- the nightsky is never black
Take care
Slawomir