Well, let's see -
The deflector dish appears on the Mike Minor renderings of the ship, and doesn't appear to be quite flush with the front of the engineering hull. In fact I can't find anything
in Jefferies' drawings indicating what his treatment of the dish would have been other than as Minor represents it, so - as Capt April said of the warp grilles - I have to do something
with it. Minor's painting represents the best info I had available, in this case. There's certainly nothing that I'm aware of to indicate that they were going to light up the deflector ala TMP.
And yeah, the grilles are purple because it was either purple or blue and I'm sick of blue - there's a lot of dark or black detailing on the color renderings that just doesn't work.
Nacelle caps - now, there's something that will need to diverge from the color renderings but I haven't decided how yet. The problem with them as rendered is that the extremely dark blue/black treatment causes the caps to simply disappear into the black background of space. I don't know what they were thinking, there - a similar color treatment was applied to the main deflector dish. I've decided to eventually represent the dish as steel or a dark gun-metal color, but as far as the nacelle fronts are concerned...I dunno. I may try a variation of the unlit, blood-red caps from "The Cage."