The translucent selection rectangle is the box you see when you left click and hold, and then drag the pointer over items to select them when you release the left click.
The border color of the selection rectangle is solid blue by default.
The color inside the selection rectangle is a translucent version of the solid blue color by default.
If you like, you can change the border and/or inside color to the same or different custom color(s) of your choice.
This tutorial will show you how to change the color of the translucent selection rectangle for your account in Windows 10 and Windows 11.