Neil is correct as usual 🙂
Gray disks are machine specific disks with hardware drivers for that model of Mac only, they include the free iLife usually.
Since your machine is 10.5, and Apple no longer provides gray disks for 10.5.
Only via Apple Sales via phone can one order the 10.6.3 white retail disks from Apple now.
If they don't have any, then search online like at Amazon.com for a disk like this
If you want to upgrade, backup your files off the machine first and disconnect.
Most commonly used backup methods
Stick the disk in and run the upgrade installer. 10.5 will be replaced with 10.6, you will need to Software Update fully to get all the security updates.
If your machine slows down aferwards,
1: Check your RAM meets 10.6 amount specifications, add more yourself Curcial.com, OtherWorld Computing, Kingston and many other places have checkers that you enter your machine and it gives the correct RAM specs and upgrade options.
install videosand tools here
http://eshop.macsales.com/installvideos/
2: you can decide to erase everything and install 10.6 fresh
How to erase and install Snow Leopard 10.6
3: Or you can perform this clone and reverse clone optimization step which keeps everything, apps and files too.
How to properly defrag a Mac's hard drive
If your doing this to get to 10.7 or 10.8, I don't advise a 10.5 era issued machine do so, it will slow down quite a bit and won't run your PPC based software of 10.5/10.6.
So there is a huge hidden cost upgrading to 10.7 or 10.8 if your machine meets the specifications.
Anyway, read thtough this User Tip here, I have links and everything for you to make a informed decision.
Things to consider before upgrading OS X
