Update your video adapter drivers
and did u install any game recently which might have altered ur settings ?
You may see that the color also became 4-bit and you can't change it. I've also had this problem but please check that your display drivers are in working condition and also your graphics card is fitted in properly.
Long time ago I had the same problem as you...
Probably some files that were related to your graphic card drivers got corrupted.
I solved mine by reinstalling the graphic card drivers...try it because I really suspect that's the problem.
Are you running Windows Xp? If so use the RESTORE feature to restore your pc to the date when ur display was fine.
1) click start>>run>>type restore and hit enter
2) a folder will open ..dbl click rstrui.exe to start the process
3) then click "resore my pc to earlier time"
4) on left side choose the date u want to resore , click next and follow .
Please Note its reversible process means you can undo the restore if it didnt solve your problem.
if you are not running windows xp...How are the graphics coming fine?, quality is fine? ..if not as few said you will need to reinstall the graphic drivers. But must be worth trying the resore feature first if you are on xp.
DO NOT UPDATE DRIVERS NOW!-RESTORE THE ORIGINAL FIRST
like any laptop yours allso came with its drivers restore disc-use it to restore your display card driver.
if it wont work go on safe mode and chek for spywhare although I'm allmost sure that the driver restore will work.
make restore it to an earlier time from control panel
Thanks all for your suggestions... Now Its proper... I just updated the drivers file which Dell gave by default ...after that my winxp os itself asked that display is not proper...want windows to automatically repair !
I just said yes ! and it got solved.
Thanks anyway for your time !