For most, the first number is the company, second is the platoon, and third is the vehicle within the platoon.
Usually listed as 123, 1st Co., 2nd platoon, 3rd tank.
In some units, later in the war, they numbered the individual vehicles 5,6,7,8,9, to attempt to throw off enemy intelligence. 5 = 1, 6 = 2, 7 = 3rd tank in the platoon, etc.
Various numbers for company HQ units, e.g. 201, 202, or 204, 205 – 201 and 204 would be one of the vehicles in the two-vehicle company HQ, as would 202, and 205. Sometimes in order, sometimes in reverse order, e.g. 204, or 205 could be the company commander's vehicle, and the other his adjutant.
Occasionally, you will see a I01, or II02 – those correspond to Battalion HQ vehicles – I = 1st Battalion, II = 2nd Battalion, and then the vehicle within the unit.
Sometimes letters precede the numbers as well.
S preceding a number usually signifies a Schwere (Heavy) unit, e.g. a Tiger tank battalion.
Colors for the numbers could vary considerably, by unit, theater, and time period, e.g. solid white, red, or black numbers, colored or black numbers outlined in white, or yellow, etc. Some numbers were also just outlined in a specific color.
You will need to know the unit, theater, and time period to narrow down appropriate schemes for your vehicles, if you want them to be accurate for a certain unit and battle.
I hope that helps.