I am NOT the person to ask for advice about which kind of plants you should put in a container garden. However, because design principles are universal (not just for interiors!) I think we can safely apply them outside to create beautiful pots of flowers to place by your front door.
Here are a few design principles at work in the flower pots by the front door in the Room of the Week:
SHAPE: The plants in the containers create a round shape, which looks great in contrast to the straight lines of the design of the house.
COLOR: The white urns are a great contrast color to the darker gray house behind it. But, they tie in with all that white trim.
SCALE: The scale of the plants match the scale of the container.
BALANCE: By keeping the plants the same on both sides of the front door, it helps to create symmetrical balance, and is very pleasing to the eye.
Here are a few other examples of design principles at work in container gardens: