void setup()
{
    pinMode(9, OUTPUT);
    pinMode(10, OUTPUT);
    analogWrite(9, 0);
    analogWrite(10, 255);
}
void loop()
{
    int i = 0;
    for (i = 50; i <= 255; i += 10) 
    {
        analogWrite(9, i);
        analogWrite(10, 255 - i);
        delay(500);
    }
    for (i = 50; i <= 255; i += 10) 
    {
        analogWrite(9, 255 - i);
        analogWrite(10, i);
        delay(500);
    }
}