I think, as Dennis's renderd prove, it's a much better looking ship than some people give it credit for. It's not as beautifully refined as the refit we got—it's the last 20% that makes or breaks a design—but still nice ship.
From the renderings I've seen, and Minor's painting, above, it looks like the nacelle caps were intended to just be the hull color.
HOWEVER, it appears Minor kept reworking the painting as the show developed, and even refit-ized it once.
Click HERE for a bigger image
As it happens, I just found a magazine which features one version of the Mike Minor rendering from 1978 that looks a lot like the one posted by Dennis (previous message), but without the whoosh lines, and with an interesting detail that I haven't seen clearly elsewhere.
EDIT: here it is
A higher res scan is HERE: http://farm3.static.flickr.com/2082/...a6e79d_b_d.jpg
The scan, unfortunately, doesn't show the relevant bits as clearly as they appear on the original magazine pages, despite my best attempts. But the lines around the nacelle caps and the major lines on the inboard warp grills appear to be painted the same way as the glowing windows and lights on the rest of the ship. There's no black around the grilles, as on the TMP refit. Whether this means they were intended to glow, or if it was just sort artistic florish, I can't tell you.
The deflector dish, while in shadow, has a bluish gray cast to it in this painting.
I also noticed there's no NCC number or any of these pre-TMP design renderings. My guess is, when they decided to put the grilles on the outboard sides of the necelles, thye noved the registry to the secondary hull and deliberately left it off the nacelles.
Make of all this what you will.