I can't get grub menu to show up during boot

I was having the same problem on my laptop, getting the magenta blank screen. Here's what worked for me. I changed:

GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true

to:

GRUB_HIDDEN_TIMEOUT=10
GRUB_HIDDEN_TIMEOUT_QUIET=false

And since I didn't have a CRT, instead I changed:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to:

GRUB_CMDLINE_LINUX_DEFAULT=""

Then ran update-grub. On restart I was then able to get the menu by holding down the Shift key.

To show the menu only when you need it

Hold SHIFT from when you see the BIOS load screen. The GRUB menu should show up.

To always show it

Run Gedit as root (gksu gedit), and open the file /etc/default/grub. There should be something like this in there:

GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true

Change it to this:

GRUB_HIDDEN_TIMEOUT=10
GRUB_HIDDEN_TIMEOUT_QUIET=false

Save it, run sudo update-grub from a terminal, and reboot. The GRUB menu should show up.

None of the above suggestions worked for me. So I tried "commenting" the two entries. i.e. I changed them from

GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true

to:

#GRUB_HIDDEN_TIMEOUT=0
#GRUB_HIDDEN_TIMEOUT_QUIET=true

I now get the grub menu screen for 10 seconds.

GRUB_HIDDEN_TIMEOUT controls whether or not the menu is shown. The default behavior is to hide the menu if only one operating system is present. If a user with only Ubuntu wishes to display the menu, place a # symbol at the start of this line to disable the hidden menu feature. For more information, see the Grub 2 Ubuntu help page