Help with TinyDB

i Guys! It's time for me again to ask some questions to you.

I have created an App, here is the layout of the app.

My question for you is how can I use the TinyDB function to save the pressed button (when the button is green) to be green when the application is turned off and turned on after a while. It's just not working...

I attach a picture of my blocks to.

This is how I want it to work:

When i press a button the button will be green to figure that i've pressed it.

If i want to press another button the first button will be black again and the previous button will be green.

When press the red button the green button will be black.

All this features I want to be enabled even if the application is turned of and then on again. 

Thank you Guys for all help, Im new here and Im sorry if you're think I'am stupid.

You just need to save WHICH button was selected when you close... Then when you open up, restore THAT button to the back color and all the rest to black except for the last which is red...