#define RED 10
#define BLUE 11
int a, b;

void setup()
{
  pinMode(RED, OUTPUT);
  pinMode(BLUE, OUTPUT);
}

void loop()
{
  for (a = 0; a <= 255; a += 50)
  {
    analogWrite(RED, a);            
    analogWrite(BLUE, 255 - a);    
    delay(1000);                    
  }

  for (b = 255; b >=0; b -= 50)
  {
    analogWrite(RED, b);            
    analogWrite(BLUE, 255 - b);    
    delay(1000);                    
  }
}