求助，本人编程小白
以下代码为何在矩形框外部颜色没有恢复背景色，求助

void setup(){ size(600,400); background(100); smooth(); stroke(255); fill(100); rect(50,100,100,200); rect(250,100,100,200); rect(450,100,100,200);}void draw(){ if(mouseX>50&&mouseX<150&&mouseY>100&&mouseY<300) { fill(0,255,0); rect(50,100,100,200); fill(100); rect(250,100,100,200); rect(450,100,100,200); } if(mouseX>250&&mouseX<350&&mouseY>100&&mouseY<300) { fill(255,255,0); rect(250,100,100,200); fill(100); rect(50,100,100,200); rect(450,100,100,200); } if(mouseX>450&&mouseX<600&&mouseY>100&&mouseY<300) { fill(255,0,0); rect(450,100,100,200); fill(100); rect(50,100,100,200); rect(250,100,100,200); }}

我想的是缺少一个else语句，可是在嵌套一个if在末尾加上else填充100，还是和以前的样子一样