Processing sketch to change alpha of a color programmatically.

AlphaColorChange.pde

color alphaColor;
// I test this program using red color.
// But you can use any color you want.
color mainColor = color(255, 0, 0);
// The least alpha value is 0 while
// the most alpha value is 255.
// This variable is the starting alpha
// value.
int alphaValue = 255;


void setup(){}
void draw(){

    background(255);
    // This is the magic using bit shifting.
    alphaColor = alphaValue << 030;
    fill(mainColor & ~#000000 | alphaColor);

    // For this example I will create an ellipse
    // that has its color change gradually.
    ellipse(
        width/2,
        height/2,
        width/2,
        height/2
    );
    // Change the alpha color (opacity) gradually.
    alphaValue --;

    // Turn the fill color back into default.
    fill(0);

}