tracking base color is unnecessary as you are passing it into your method.
this is a simple way of make the color progressively darker
public void createCircles(int x, int y, int rad, Color parentColor){
Circle myCircle = new Circle(x, y, rad, parentColor);
...
if(!(rad<1)){
...
Color myColor = parentColor.darker();
createCircles(x - (2*rad), y, rad/3, myColor);
createCircles(x, y, rad/3, myColor);
createCircles(x + (2*rad), y, rad/3, myColor);
}
}