Only cubes as boot option on boot

I just installed CubesOS on a separated encrypted drive.
Why don’t my other OS don’t show up in the boot menu?
And my boot menu is all black/white unlike what I saw on boot on other people’s install.

You mean from Qubes OS GRUB menu?
By default only Qubes OS is present in your GRUB config. If you want to add your other OS to the Qubes OS GRUB menu then you need to do it manually.
Or you can use UEFI to boot your OS from another drive instead of using Qubes OS GRUB menu.

I think black/white is on UEFI systems and colored is on Legacy BIOS systems. Or was it the other way around? In any case it shouldn’t matter.

The black and white GRUB menu is normal since R4.2 (both for UEFI and for legacy BIOS systems):

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If you’re on EFI, take a look at efibootmgr to restore your other OS’ EFI partition. If doing that from Qubes isn’t easy, try live Ubuntu from a CD/USB. My experience was that the Qubes installer didn’t touch the boot/efi partitions, just removed the EFI entry from the firmware config so I couldn’t see the option in the UEFI interface.

You could also try and do it through grub, but that’s likely to undo itself every time there’s a kernel update unless you know what you’re doing.