Why use a container field for this?
The RGB function must be used with the TextColor function to specify text color. It can't to my knowledge be used to change the fill color of a field.
But a conditional format expression can be specified to give any type of of field, container or otherwise, different fill colors. But an image inserted into a container field will conceal much of the fill color--possibly all of it, produced by the container field.
I am not using images and you just explained so much. Can I ask how you call the FILL colors out to be change in the expression or equation?
Place a field on your layout.
Right click it and select Conditional formatting....
In the dialog that appears enter an expression that evaluates as True or False. ( (CurrentDate-StartDate) > 30 ) Use the controls in the dialog to select the field formatting options--which includes the fill color you want for when the expression is True.