The simplest possible usage of strtol...
void setup( void )
{
Serial.begin( 115200 );
}
void loop( void )
{
char buffer[20];
long value;
strncpy( buffer, "0xFFDD00", sizeof(buffer) );
buffer[sizeof(buffer)-1] = 0;
value = strtol( buffer, NULL, 0 );
Serial.println( value );
delay( 1000 );
}