I think the best list about original 2600 colors is still Klaas Dijkhuizen's spreadsheet attached to this post.
This whole thread (not just my post) is also quite informative. I have yet to see any documentation that a 2600 Spider left the factory in silver metallic.
IMO, the silver mettallic paint was introduced in 1965 on the Giulia GT and 2600 SZ -- by about the time that Carrozzeria Touring went out of business. So, no, very likely not an original color -- but doesn't mean you shouldn't paint it that way if you like it!