When you hold the mouse button, the ellipse is drawn in black, otherwise it’s white. This is the fill color, not the stroke color (the outline).
It looks better, when the outline is not black but darkgray
void setup() {
size(480, 480);
}
void draw() {
stroke(111);
if (mousePressed) {
fill(0);
} else {
fill(255);
}
ellipse(mouseX, mouseY, 80, 120);//
}