Thats a pretty limiting colours, you can use actual images for the splash too.
WARNING if you dont know what you are doing dont mess with it, you could mess up your system. First locate this entry in your /boot/grub/grub.conf or menu.lst (you will need to be root).
splashimage=(hd1,0)/boot/grub/splash.xpm.gz
If its there then good if not then make one just at the top of some commands. Make sure you are not breaking any continued command on the grub list.
1. The Splash.xpm is 'bmp' of Linux an INDEX(16Colour) image format(No transparency).
2. Copy the file from /boot/grub/splash.xpm.gz (If it exsits).
3. Make a seperate copy for backup.
4. Fire up GIMP and open that splash.xmp.gz file. (Note you can append .gz to any file while you are saving it from gimp even .bz2)
5. Copy another splash or make your own and paste it into the splash.xpm.gz file. Save it.
6. Go to Image>> Flatten.
7. Go to Image>>Mode>>Index.
Here change the colour from 256 to 16.
8. Replace the old new splash.xpm.gz with the new one.
(NOTE if that file dosent extsts then simply save any file in the same format .xpm.gz)
Restart and watch a vibrant boot up screen.
Cheers!
UPDATED: A Minor mistake