Originally, the flag is only a simple ribbon tied on the pole.
The ribbon is the same with all different form of team bases. It is a greenish-white ribbon tied on a white pole.
After Renovation, it changes to a different pattern and shape, depending on which species the Starter Pokemon is.
Notice that the pattern somewhat is similar to the starter Pokemon's pattern/body part (For example, Eevee's flag has a similar pattern to its tail, and Squirtle's has a shell on it.)
After recruiting the Smergle from the howling Forest mission, when the player talks to him in his Friend Area, it will give the option of re-painting the flag. If the player selects 'yes', the flag will be in a different pattern the next day.
After going through all the different variations, when the player tells Smergle to re-design the flag again, he will say he is out of ideas and gives the same flag after the renovation.
They are all rectangular in shape.