@begueradj Go into /etc/default/grub and change GRUB_TIMEOUT=10 to say GRUB_TIMEOUT=3, or whatever timeout you like and then run update-grub. You will then have permanently changed the Grub Timeout.
In theat same /etc/default/grub file, GRUB_DEFAULT=0 determines which is the default OS to boot. 0 being the first line, 1 being the second line etc.. If you change that, then run grub-update, it will change that option. That is, until you add another OS or update the kernel and get a new entry on top of the grub list. Manually changing options requires you to keep an eye on how those changed options effect grub when new items are added.
Changing the colors is a bit more complicated, so, I will have to look for the link to the how-to. I'll get back to you on that. Meanwhile, here's a link that might be helpful for you. It adds a bit more to the grub2 explanation: