Download a profile for any reflective - not too familiar with the jv300 or it's profiles... kind of surprised there isn't one for 680 since it's popular. But some common ones to look out for -
Oralite 5600, 5700
Nikkalite 4801
3m 5300, 3290
Avery v4000.
I'm sure one of those will have a profile...
Yes, the colors change a bit since it's a silverish reflective. It should not be noticeable enough to cause an issue unless you're trying to get a very specific pantone match. 99% of the time the color is close enough to not be an issue. But yes.. I'd you print the same blue on reflective as on white and hold it side by side, it won't match - but even if you have a dead accurate profile, that'll still be the case... As light hits the reflective in different angles, the color changes / shifts slightly..
Obviously print a small sample if it's your first time printing on the material to make sure it's laying down the right amount of ink and not artofcating, and the colors aren't too off... But printing on 680 is pretty much as easy as printing on white vinyl... It just costs 5x as much when you screw up.