I want that a cell background color will change upon calculating a value.

That is to say, if the calculated value is greater than 1.2, then the
background color of that cell will change to red. Conversely, if the
calculated value is less than 4.0, then the background color of that cell
will change to yellow.

The colors which I want to utilize are red, green and yellow.

Thank you in advance for your assistance.