Oops, somehow missed this comment.

Here’s the symbolism (copied from Reddit):

American Samoa

• The triangular pattern is taken from the existing flag, just flipped to be more prominent when waving.
• The red, white, and blue symbolize American Samoa’s link to the United States.
• The color arrangement—a field of red with a blue shape on the left and white symbols within the blue—is similar to that of the Samoan flag.
• The war club and fly-whisk are symbols of Samoan leaders.

Guam

• The blue field stands for the vast ocean surrounding Guam.
• The red symbolizes the violent history of the island.
• The two shapes are meant to resemble a slingstone and a sinahi necklace, both Chamorro symbols.
• Yellow is used to represent the hope of a prosperous future and so that the center of the flag resembles islands in the sea.

Northern Mariana Islands

• The blue once again represents the ocean.
• The white slopes on the sides of the flag evoke the Mariana Trench, which lies near the Mariana Islands.
• The latte stone in the center of the flag is a Chamorro symbol.
• There are 14 stars for the 14 islands in the territory. The red and pink resemble that of the original flag’s wreath, a symbol of the Carolinians.

Puerto Rico

• The white stripes are now yellow to better symbolize Puerto Rico’s Spanish history.
• The rest of the symbolism is the same.

U.S. Virgin Islands

• 'Murican colors represent 'Murica.
• The Nordic cross alludes to the islands’ former status as a Danish colony.
• There are 3 stars for the 3 main islands in the territory.