Color color1 = new Color(66, 241, 244, 255), color2 = new Color(139, 40, 130, 255), current = null;

    int maxSteps = 150, i = 0;

    ColorInteger r = new ColorInteger(color1.getRed());
    ColorInteger g = new ColorInteger(color1.getGreen());
    ColorInteger b = new ColorInteger(color1.getBlue());

    double diffR = color1.getRed()>color2.getRed() ? (color1.getRed()-color2.getRed())*-1 : color2.getRed()-color1.getRed();
    double diffG = color1.getGreen()>color2.getGreen() ? (color1.getGreen()-color2.getGreen())*-1 : color2.getGreen()-color1.getGreen();
    double diffB = color1.getBlue()>color2.getBlue() ? (color1.getBlue()-color2.getBlue())*-1 : color2.getBlue()-color1.getBlue();

    double stepR = diffR/maxSteps;
    double stepG = diffG/maxSteps;
    double stepB = diffB/maxSteps;

    while(true) {
        if (i==maxSteps) {
            i=0;
            stepR *= -1;
            stepG *= -1;
            stepB *= -1;
        }
        current = new Color(r.increaseAndGet(stepR), g.increaseAndGet(stepG), b.increaseAndGet(stepB), 255);
        window.setBackground(current);
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        ++i;
    }