How to reset default screen settings?

If resetting PRAM doesn't work, you can consider resetting NVRAM (think of the two as similar, except resetting PRAM is using a hammer and resetting NVRAM is using a sledgehammer).

Restart holding down the command-option-o-f keys; at the "Welcome to Open Firmware" black text on a white screen type in the following commands and press return after each (case sensitive, no spaces):
reset-nvram
set-defaults
reset-all
The Mac should restart after reset-all (if it doesn't, type mac-boot and press Return)

Another possibility: creating a test profile, just skip over the step where you can't see the lones (accept whatever value it comes up with) and play with adjusting the following steps, such as the color temperture and gamma settings.

Good luck...

Elizabeth Pass1,

Since display settings are stored in PRAM (see Mac OS X: What's stored in PRAM? , try resetting PRAM by restarting while holding down the command-option-p-r keys until you hear three sets of chimes, then release the keys and allpw the Mac to finish rebooting.

BTW, I recall having similar problems not being able to see the image in the display setup; it may help to reset the System Preferences> Displays> Color tab settings to one of the default profiles before trying to calibrate anything.