Change an element on a different page?

Sorry if this is a super basic question. I am trying to make a really simple app just for our own small business use and am very, very new to this.

Essentially I have one page that is a grid (all red squares). When I click on one square of that grid I want to go to a new page where I can input data about that square. At the bottom of my data page I want two buttons. Button 1 takes me back to the grid page (I got that one). Button 2 takes me back to the grid page but the square that I clicked on to open the data page is now turned green. My problem is I can’t figure out the workflow on how to make an element on my data page (button2) change an element on my grid page (make the red square a green square).

Anyone that can help me?

If you input data for a square and you saved it in a table (Create Thing), you s/b able to use that data to change the colour of the square.