Outside of the actual paint scheme, you really do need to mark your vehicles.
As I set about cutting the grass this past weekend, I decided that I needed to add flames to my lawnmower. A rather spur of the moment thing, but it had to be done. It took about 15 minutes total (including clear coat) to get this finely decorated war machine ready for battle.
I can't look at it without smiling and being so proud of it.
My Wife was not impressed.
It's one thing to get your vehicles painted up in 40k. It's another to go that next step and tie them in with your force. Instead of just fielding a blue Ultramarine Rhino, why not assign it to a squad and give it the appropriate markings and numbers?
I'm not even getting into weathering here, just a few simple numbers and such to add some character to your vehicles. I've talked about it before with when to do freehand and how to do it posts.
You don't have to go all out either. Nothing says you need to cover your vehicle in freehand for it to look good. A simple arrow or design on the top can do wonders. Don't get me wrong, a nice freehand design can absolutely change the look of a vehicle, but it's not a game breaker.
Sure this guy might look really nice, but he's going to draw a substantial amount of firepower putting that Armour 14 to the test. Even if it's empty, your opponent may want it destroyed just so you can't parade it around the table showing it off.
Whether you add a simple icon to the side doors or some basic freehand, it will go a long way towards giving your vehicles a "finished" look and bring them into your force in a unified way. It's a big return for a small investment.
As soon as I finished painting my lawnmower, it broke and I had to replace the spark plug. But... it does go faster now.
Make sure to check out these posts as they might help:
Painting tactical squad arrows on Rhinos