Create coloured cells depending on value of cell

Hi

I am trying to make excel colour a certain cell depending on how many months arrears a tenant is in.

Aiming to create a traffic light system i.e those who are in line with their payments cells will be coloured green, those who are in arrears of less than 1 month will be yellow. Those who are 1 month and below 2 month will be amber and those who are more than 2 months in arrears will be red.

I have produced a mock workbook which I hope will explain this more effectively.

Any suggestions will be welcomed

Many thanks

Will