Hi o_o Yao,

Try adding a background right after you open the draw function. If this doesn't work, could you give me a link to the project so I can take a look at it (don't link me to the challenge. It will display as my own code for the challenge. Either give me a link to a spin-off, or create a new project and paste your code into it.) 

Thank you for answering my question.

Before you answered, I'v tried a method that is to put mouseClicked function to each Scene and use if inside the mouseClicked function. that did work ,but the button didn't change color when I put my mouse on it(I'v defined the Button.draw to make the button change color.)

https://www.khanacademy.org/computer-programming/this-is-not-finished/6288549450383360

This is the pre-program I'v made.

So right now, you're trying to make the buttons change color when they are hovered over?

Mainly. But the way I make buttons appear is to troublesome. I want to find an easier way.

Beside. I'v tried draw in each drawScene. It did change the color, but the previous button appears on the next scene.

The issue is that you don't have a draw function. Put line 969 inside of the draw function and the program should work.