My favourite conditional format is one we use in our invoicing system that highlights the old invoices that haven't been paid. This shows invoices that are a month overdue with a purple box, invoices that are 6 weeks old with a red box and invoices that are 6 weeks old where we have had a follow up conversation with a yellow box. See this in figure 1. In figure 2, I illustrate the conditional formatting expressions for the coloured box.
To Make the Colour Box Happen
To make a colourful highlight box as in figure 1, put the Forms index field in a (second) text field and lock the field. The conditional format will need to show Red text on a Red background for the Red condition as in Figure 2. If no conditions are met, make the box and text the background colour of the form.You can work out the rest.