How to buy vehicle gauge colors and apply? -> Ingame, Type /shop -> Vehicle Upgrades -> Gauge colors and buy a gauge of your preferred color.After the purchase is successful you'll get a message in dialogue box stating your purchase has been completed and your item is available in /purchased. Get inside the vehicle in which you want to add the gauge color you've purchased and type /purchased and select "Gauge color - x" to add it in your vehicle.
Please do not forget to save the vehicle after upgrading it. Also, please note that you can buy only one gauge color at one time and after applying the first color you may purchase another one and apply.If you do not apply to any vehicle, your previous gauge color gets replaced by new one in /purchased list itself.
Here are the available gauge colors along with their looks in-game for your ease :-
You can use command /hud to switch to old speed-o-meter.