Your context for wearing the topcoat should be the determining factor of style and color. Scenarios 2 & 3 are about the same so you have, A) worn with a suit or B) worn casually over casual attire. As others have said, few colors and styles fit both needs. Decide which scenario to dress for and get by with this coat for the other situation until you can add an appropriate coat. If you dress predominately for or 2 & 3, don't get navy. If it will mostly be worn with a suit, get navy. If worn for each situation equally, get grey. There are more choices of various weaves and textures in grey than blue and picking the appropriate finish will bring versatility.
I haven't owned a navy or midnight topcoat. Have a dark grey, not charcoal, and it is a casual raglan style with slash pockets and lighter weight. Had a dark plum color, venetian cloth, cut as a trench coat that was amazing. Wore it out. My dress coat is a very dark brown and black herringbone ( you can find this cloth in most of the outerwear sample books, particularly Holland & Sherry , Scabal) cut as a DB Chesterfield, black velvet collar. This coat goes with every color suit except brown. Have several coats in shades of brown, tan, vicuna in solids and patterned cloths. These are worn the most.
Have two in the works, a grey plaid and navy twill. Both are double faced and will be unlined. An experiment. Want to remake the brown/black herringbone chesterfield as the one I have is from 1985. and looks a bit aged. Also have a dark bottle green barleycorn pattern cut as a raglan, not in a hurry to finish it. Have two mid brown and cream large herringbone topcoats cut. One is a raglan with patch pockets and the other will be a SB Chesterfield with a dark butterscotch velvet collar. Liked the idea of both styles so I am making both. Thought it would be cool to see two totally different interpretations of the same cloth. Another experiment.
Have a light navy herringbone that will probably be made up as a SB Chesterfield with a navy velvet collar but that is for a later time.
At some point I want to make a light colored gabardine as a trench coat or maybe a wrap. Then a classic Polo.
I have a thing for topcoats.