There is a slight difference in the two colors. Meridian was used in '68 and is slightly 'brighter'...that is, more silvery turquoise than Crystal was.
Crystal is the '69 color and is just slightly deeper in the blue/green tone.
Hard to describe but the bottom line is that Meridian is the correct one for '68.
The link posted above by RR has lots of examples but they are not totally reliable as the administrator over there takes the word of whoever submits the photo as to what color it is...therefore, the examples range all over the place.
The fact that cameras give various results doesn't help either.
Below is a photo of a local '68 converitble. Although it's a '68 the paint is '69 Crystal Turquoise. I was told this by the guy that restored the car.
And here is my car before I stripped the paint off
You can see that they are very close but the Meridian is slightly 'brighter' in hue. Both nice colors though.