Plymouth is an application (bootsplash) that runs very early in the boot process (even before the root filesystem is mounted!) that provides a graphical boot animation while the boot process happens in the background .
By default ROSA comes with minimalistic plymouth theme "Rosa-EE" (background colors may vary depending on ROSA edition).
If you wish to change it for some other theme you should do the following (will take the bluish MIB Ossigeno Ultimate Plymouth theme as example):
1) download the theme archive
2) extract the folder and copy it as root in /usr/share/plymouth/themes (the full path will be /usr/share/plymouth/themes/MIB-Ossigeno-Ultimate-Plymouth)
3) remove the folder /usr/share/plymouth/themes/Mandriva-Rosa or rename it ('Mandriva-Rosa.old' for example)
4) insert as root
plymouth-set-default-theme MIB-Ossigeno-Ultimate-Plymouth /usr/libexec/plymouth/plymouth-generate-initrd /usr/libexec/plymouth/plymouth-update-initrd
Reboot and enjoy watching :)
To completely disable plymouth theme, edit /etc/default/grub file (root permissions are required) and remove quiet and splash parameters (in particular, splash=silent) from GRUB_CMDLINE_LINUX_DEFAULT
After that, update GRUB2 (with root privileges, as well):
# sudo update-grub2