I have created a device that could adjust the LED brightness according to the environment brightness captured by a sensor and with schematic, circuit, LEDs, analogRead(), analogWrite(), the map function, and if-statements. I built it with Arduino and implemented it on a breadboard.

1. I have first calculated the appropriate resistance for each LEDs. The red and green LED have a same voltage drop. I calculated them at the same time. I have also calculated the proper resistance for the Photoresistor.

2. Then with the value of resistor, I have drew the schematic for the circuit.

According to the schematic, I have built the circuit with Arduino on the breadboard.

          
            const int analogInPin = A0;  // Analog input pin that the potentiometer is attached to
            const int analogOutPinGre = 9;  // Analog output pin that the LED is attached to
            const int analogOutPinRed = 10;  // Analog output pin that the LED is attached to

            int sensorValue = 0;  // value read from the sensor
            int highSensorValue = 0;  // value will be the highest value from sensor
            int lowSensorValue = 1023;  // value will be the lowest value from sensor
            int outputValue = 0;  // value output to the LED (analog out)

            void setup() {
              // initialize serial communications at 9600 bps:
              Serial.begin(9600);
            }

            void loop() {
              // read the analog in value:
              sensorValue = analogRead(analogInPin);
              /* check if the current sensor value is higher than 
                previous created highSensorValue */
              if (sensorValue >= highSensorValue) {
                // if yes, replace the highSensorValue with current sensorValue
                highSensorValue = sensorValue;
              /* check if the current sensor value is lower than 
                previous created lowSensorValue */
              } else if (sensorValue < lowSensorValue) {
                // if yes, replace the lowSensorValue with current sensorValue
                lowSensorValue = sensorValue;
              }
              // map it to the range of the analog out:
              outputValue = map(sensorValue, lowSensorValue, highSensorValue, 0, 255);

              // print the name of output to the Serial Monitor:
              Serial.print("sensor = ");
              // print the current sensor value to the Serial Monitor:
              Serial.print(sensorValue);
              // print the name of output to the Serial Monitor:
              Serial.print("lowsensor = ");
              // print the lowest sensor value to the Serial Monitor:
              Serial.print(lowSensorValue);
              // print the name of output to the Serial Monitor:
              Serial.print("hisensor = ");
              // print the highest sensor value to the Serial Monitor:
              Serial.print(highSensorValue);
              // print the name of output to the Serial Monitor:
              Serial.print("sensor = ");
              // print the mapped output value to the Serial Monitor:
              Serial.println(outputValue);

              // change the analog out value of the green LED:
              analogWrite(analogOutPinGre, outputValue);
              // change the analog out value of the red LED:
              analogWrite(analogOutPinRed, (255-outputValue));

              // wait 2 milliseconds before the next loop for the analog-to-digital
              // converter to settle after the last reading:
              delay(50);
            }