Just one number isn't enough.
I have experience only with sprit's springs, in which case 68g is supposed to be about the same as vintage MX Black and 78g is a lighter variant of stock MX Clear spring. The former is easier to bottom out, but the latter requires about as much or perhaps a tad bit less force at actuation. In other words, if stock modern MX Black is 80g at full compression, it's about the same at the 78g sprit's spring, but not actuation (78g is at least 5g lighter in that point).