How do I really know that my juice is Syrah, and are there Syrahs that don’t have that deep purple color?

You say you have a Syrah that is not purple, and how do you know it’s Syrah? To address your second question first, I would say you really have to trust your supplier unless you want to spend a lot of money playing detective (more on that presently). The first step I would take is to look at the package and/or the packing slip that came with the grapes . . . I hope they say Syrah. If you still doubt the paperwork, certainly get in touch with your juice supplier and ask to see the paper trail from the lot of juice you received. Your supplier should have records of the entire process, from harvest to sale, showing the grapes being brought into the juicing plant from the field, being processed and packaged up into the containers you received and then the packing slip going to you. It might take a little time and phone calls on your part and a little annoyance on their part, but if they are on top of their customer service game they should