Because if you apply for a trademark in black and white, you can change the meaning at will when using it

If you apply for a specified color, the color when you use it must be consistent with the color when you apply.

What does black-and-white draft (grid diagram) mean in LOGO design?

Black-and-white proof refers to removing the color of the color proof and making a black-and-white draft. Black and white coordinates, in order to facilitate merchants to accurately draw the LOGO, a designer is required