Nice image, cool colours.
Eliminating purple stars is easy - The general concept is to turn all the stars white with PhotoShop's colour noise reduction routine.
http://bf-astro.com/starColors.htm
PhotoShop's Reduce Colour Noise is used to gain this effect. On the Reduce Noise Basic Screen, set the filters "strength" to zero, "reduce colour noise" to 100% and "sharpen details" to zero and press OK. The noise reduction routine seems to identify the stars as colour noise and will set the stars to white. You may have to re-execute the filter a couple of times to get them really white. Press "Ctrl+F" to run the routine again.
Then the intensity of the white stars can be reduced by using PhotoShop's Image|Adjustments|Selective Colour routine. Select "White" in the colour drop down box. Then reduce the brightness of the white by moving the "Black" slider to the right, probably all the way, and click OK.