From #9410 Cormyr, 'Local Militias', p.58:
Each militia banner is half Purple Dragon and half the town's own banner, divided per fess (vertically).
I take this to mean that—within reason—each village, town, and city in Cormyr has its own blazon. However, no where in Cormyr does it give any indication what blazon each one uses. So for Eveningstar I made up my own—a white star on a deep blue field, and IMHO it fits quite well. Regarding the artwork itself, I based the Purple Dragon on the one drawn by the talented Valerie Valusek in FRQ1. Believe you me, it was no picnic producing this figure at the pixel level with my graphics program at full close-in zoom!
Now a correction to the text from Cormyr quoted above. Per fess is not a vertical division of the field but a horizontal one. Per pale is the proper term for a vertical division, and since the author placed 'vertical' in brackets I assume that he did in fact intend the division to be vertical but just got the terms mixed up.
On another note, IMC each member of Eveningstar's militia has a suit of studded leather armor, over which they wear a tabard (half white, half blue) that displays Eveningstar's arms as above.