My mac powerbook g4 12'' wont chime or boot just comes on and screen changes colors every 2 seconds

The parameter ram reset will reset that and other basics back to factory without losing any data. To do this, take a moment and find the command, option, p and r keys and figure out how you are going to hold them ALL down at together. The mac should be off, if not, shut it down (not restart-shutdown). Now, touch the power button and immediately hold down the cmd-option-p-r keys. Hold them for the screen to cycle black to gray THREE times (each taking about 15 seconds), then let go and let the mac complete the boot cycle. On the second cycle, you should have already heard the "chime" appear. That's it! Good Luck.

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If there's no chime, your problem could be anything from a dead battery or bad AC adapter to a dead logic board.
Does the tip of the adapter on your AC connector light up? If not, that's a problem.
When you push the button on the battery, do you get at least two glowing LCDs? If not, that's a problem.
Otherwise, try the following: remove the battery and the AC jack to make sure the machine is off. Now insert the AC jack, and reset the power management unit by pressing the following multi-key combination:
Shift Control Option Power-button
Wait five seconds, then press the power button to attempt to start the computer normally.
The instructions above are for the 1.5GHz PowerBook G4; the instructions MAY be different if that is not your model.

try booting with control command P and R all pressed at the same time, wait for second chime then release.

If a Mac is shut down while the volume is set to zero, the standard Apple startup chime will not sound, so it is impossible to say whether or not this symptom indicates a problem.
For a startup issue such as this, I would recommend zapping the PRAM and resetting Open Firmware.
To zap the PRAM, start with a powered-down machine, press the power button, then immediately hold down the four keys: command, option, P, R. After ten seconds or so, you should hear the optical drive reset, and then the startup chime (because the volume is reset to full by this procedure). Ten more seconds, another reset, another chime. After the second time you hear the chime, release the keys.
Shut the machine down again (if it doesn't start up successfully, hold the power button until the power goes off). Press the power button, then immediately hold down the four keys command, option, O, F. You will see some tech-speak on the screen, then it will wait for input. Enter the following three lines, pressing the "return" key at the end of each:
reset-nvram set-defaults reset-all
After the last line, your Mac will reboot itself. It should now boot much more reliably. If it doesn't, chances increase that you have a hardware problem. You may want to run the Apple Hardware Test from the DVDs that came with the PowerBook to check your memory and logic board. Failing this, you are in for a trip to the shop.

I suggest you plug it into AC power, then boot with Command-P-R held down until you hear the unit chime twice, then release it. Hopefully, it will boot after this.
If not, boot with Command-O-F held down; when you reach the command line, type
reset-nvram [return-key]
reset-all [return-key]
If it doesn't boot after that, you have deeper issues if the machine is actually turning itself on and off.

there is a hardware fault or full hdd ...

thi guys will help Micromen - London

your problem is probably on the hard drive. The chime sound is a file and as to be read on the hard drive.
Try this: reboot system with cmd+s hold. Then type fsck -y or fsck -f (too long to explain, one of those will work) .
the computer compute and says:
***** FILE SYSTEM WAS MODIFIED *****
type your fsck -y or -f again until the computer no longer says
***** FILE SYSTEM WAS MODIFIED *****
after that, you type reboot, or shut down and power again.
If this does'nt work, change the hard drive (mac shop - expensive)

Probably more like a Video cabling issue or a Screen assy getting bad connections.. Both have been seen here.

Good luck!

Thinking about this one I really think the video had nothing to do with the black screen. I would try to things one is reset the power management. Not know exactly what Powerbook you have I will give the most common way. Remove the Battery and the power cord from the powerbook, wait a few minutes then hold the power button for about 40 seconds. Then put the battery and power cord back in and restart it. The second thing to do is boot from the install disk that came with it and run the disk utility. I have also seen bad RAM do this but you would get at least a chime on startup.