Com este projeto que utiliza 3 leds sendo nas azul, vermelho e verde, é possivel obter qualquer cor.

rgb

float RGB1[3];
float RGB2[3];
float INC[3];
 
int red, green, blue;
 
int RedPin = 11;
int GreenPin = 10;
int BluePin = 9;
 
void setup()
{
    randomSeed(analogRead(0));
 
    RGB1[0] = 0;
    RGB1[1] = 0;
    RGB1[2] = 0;
 
    RGB2[0] = random(256);
    RGB2[1] = random(256);
    RGB2[2] = random(256);
}
 
void loop()
{
    randomSeed(analogRead(0));
 
    for (int x=0; x<3; x++) {
       INC[x] = (RGB1[x] - RGB2[x]) / 256;
    }
 
    for (int x=0; x<256; x++) {
       red = int(RGB1[0]);
       green = int(RGB1[1]);
       blue = int(RGB1[2]);
 
       analogWrite (RedPin, red);
       analogWrite (GreenPin, green);
       analogWrite (BluePin, blue);
       delay(100);
 
       RGB1[0] -= INC[0];
       RGB1[1] -= INC[1];
       RGB1[2] -= INC[2];
    }
 
    for (int x=0; x<3; x++) {
       RGB2[x] = random(556)-300;
       RGB2[x] = constrain(RGB2[x], 0, 255);
       delay(1000);
    }
}