Grayscale isn't working in ui like the crosshair does.
You can try it.
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Can confirm in 1.21.2.
Steps to reproduce:
Apply the attached resource pack
Load into the world. Notice that instead of "second" crosshair showing whats behind it in black and white, the crosshair is simply white.
@luojin573281 That is the Minecraft crosshair, the resource pack adds a second ui element with crosshair texture i does not replace it.