digitalWrite(ledpin = 3, LOW);

digitalWrite(3, LOW);

redledpin = 2;
greenledpin = 3;

readChar = Serial.read()
if( readchar == 'a')
...
if( readchar == 'A')
...
if( readchar == 'b')
...
if( readchar == 'B')
...

if ( Serial.available() )
{
    ...
}

A, B, 142, 090