The rigging of a sailing ship is divided into two parts. Standing rigging holds up the masts. Running rigging controls the yards and sails. Standing rigging generally was coated with tar - usually Stockholm tar, which is a dark, rich brown. Running rigging, because it had to run through various blocks and other fittings, generally was left untreated.
I'm afraid the best thing to do with the thread that came with the kit is to throw it in the waste basket. Line in plastic ship kits is notoriously cheap. For such a beautiful model as the one you're working on, I'd recommend that you consider buying some line from Syren Ship Model Company, which is generally thought to have the best in the business. A Reale doesn't really have much rigging; the necessary line wouldn't be too expensive. Here's the link: www.syrenshipmodelcompany.com/miniature-rope.php .
There's been quite a bit of argument over the years about the right colors for rigging on ship models.
If I were you, I'd pick the dark brown for the standing rigging and light brown for the running rigging. The dark brown looks to my eye like rope soaked in Stockholm tar, and the light brown looks like well-used untreated rope.
Good luck. This is going to be a superb model. As a matter of fact, it already is.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.