Everything which is non-longer valid in a UEFI environment is "greyed".
Unfortunately that's quite a lot of stuff.
MS won't allow "foreign" OSs to boot from bootmgr under UEFI (that even includes its own legacy products). Even EasyBCD cannot circumvent these absolute restrictions.
The only way to dual boot Windows with Linux under UEFI is to install Linux with the option to take control of the boot process enabled.
Then you can chain Windows from the grub menu.
Hi, thanks for the answer.
I want to start two Win10Pro versions where I can hide partitions, that's why I need the NeoGrub. Is there a way to do that? I am new with Win10, can it be installed without UEFI and would it solve that problem?
If you're starting from scratch, you can install W10 in legacy BIOS mode, but if it's already bundled on a UEFI PC you can't convert it.
The MS boot managers have never had the ability to flip the "hidden" bit as you presumably are aware since you're looking at Neogrub, though they do respect its setting.
However a much simpler way to effectively hide the existence of two versions of Windows from each other (BIOS or UEFI) is to remove the disk letter of the unbooted system on the live one using the Disk Management snap-in.
Windows (any flavour) doesn't display in Explorer (and cannot access) any drive to which a disk letter has not been assigned.
(the sole exception being the above snap-in which can of course "see" the drive in order to reassign a letter if desired.)
If each system excludes the other in this way, you can run a PC which apparently has only one OS to the end user, except at boot when both options are available in the menu.