I would actually go with the "Shapes" of the ships as a better indicator in Star Wars.
The Empire mainly goes with sharp edges and angles while the Republic went with rounded and organic shapes.
It's a little less obvious in Trek due to the WARP designs seemingly having to have paired engines which are seen on just about all the starships.
In Trek's case "Colors" would appear to be the better indicator.
Grays & Whites for the Federation, Reds for the Vulcans and Greens for the Klingons & Romulans.