The problem is that the value must be a hexadecimal value, which is not very clear in the VS IDE.
In fact, setting the background color to “black” in the application manifest will show the tiles in the IDE as having a black background, and even make the tile black background when launched as a desktop application; but when launched in the phone emulator, he completely ignored a certain background color.
Changing it to #000000
finally resolved my problem.
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