Yes indeed, Gray Discs are not only Machine specific, but IntelMac only, only the Black Retail 10.4.x Discs work on PPC Macs.
You might be able to get by with only the 1st DVD by...
Custom install, and eliminate enough Printer Drivers, Languages, Fonts, and Applications you don't need... then it may skip #2 altogether. Reboot & hold the Option/at key as well as the left Mouse button to eject Disc#2 & bring up the Startup manger to choose Disc #1 again.
I tried my black OS X 10.4.6 CDs on a G4 yesterday specifically to see if an install from disk 1 only is possible. Sadly it is not. It requires about 400 GB from disk 2 no matter how minimal you try to make it. My disk 2 contains the following and the install needs everything except languages:
Have you considered upgrading to Leopard? Here is my general upgrading advoce:
OS numbers and names
OS X 10.4.x - Tiger
OS X 10.5.x - Leopard
OS X 10.6.x - Snow Leopard
OS X 10.7.x - Lion
OS X 10.8.x - Mountain Lion
More about Macs
The Apple History site has specifications for every Mac ever produced: http://www.apple-history.com/
Upgrade to Leopard
Those wishing to upgrade to Leopard should be aware that install disks can be expensive unless you contact Apple. Details: http://lowendmac.com/deals/best-os-x-leopard-prices.html Standard Leopard installers impose several hardware limitations including speed and RAM size but all these restrictions can be overcome. Google for details. Leopard works well at 500 MHz with 1 GB of RAM and many happy users have less than this.
Upgrade beyond Leopard
OSs beyond OS X 10.5.8 require an Intel processor. If in doubt check this: Click the apple at the top left of your screen and select 'About this Mac'. This will give you your OS number. Then click 'More Info' to see which processor you have. If it says PowerPC you cannot upgrade to Snow Leopard and above. If you have an Intel Mac it is well worth upgrading to Snow Leopard now and then considering other options after that. You can buy Snow Leopard here: http://store.apple.com/us/product/MC573Z/A
Upgrade beyond Snow Leopard
Information about upgrading Snow Leopard to Lion or Mountain Lion: http://store.apple.com/us/product/MD256Z/A
Check that your Mac complies with any requirements. If you are not in the US you should use the Change Country link at the bottom of Apple pages.