How can I make this code faster in C#

/*class Car:IVehicle //the class, just to see what's needed (license plate number which is always like AAA111 - BBB222 so 3 letter and 3 number and the color) 
    {
        string lpn;
        public string LPN { get { return lpn; } }
        string color;
        public string Color { get { return color; } }
        public Car(string lpn, string color)
        {
            this.lpn = lpn;
            this.color = color;
        }
    }

List<ivehicle> vehicles=new List<ivehicle>(); //I'm using a list so that's why i'm showing that to you
List<int> penalities=new List<int>();*/

public bool Suspicious(IVehicle veh) //this method job would be if the police can't see the license plate number perfectly (first you should check if the colors are the same as you can see in the code) and with this you can check if there is a little mistake in the lpn example (in list AAA111 & red, and you run the method x.Suspicios(new Car("AAX11X","red")) it gives you back true if atleast 4 char is same in your list, so e.g. x.Suspicious(new Car("AXX11X","red")) gives back false, or if i change the color so it does give back false
        {
            foreach (Car x in vehicles)
            {
                if (x.Color == veh.Color)
                {
                    int coun = 0;
                    int i = 0;
                    while (coun < 4 && i < 6)
                    {
                        if (x.LPN[i] == veh.LPN[i])
                            db++;
                        i++;
                        if (db > 3)
                            return true;
                    }
                }
            }
            return false;
        }

//in the main
log x=new log();
x.Initiate(3);
x.NewIrregular(new Car("AAA111","red"),1000);
bool sus=x.Suspicious(new Car("AAX11X","red");//so atm it gives back true