Hi, and welcome Jon. "Signature Series" is often the term used to describe the premium 7-spoke chrome wheel package (from GM) that came with selected Rivieras, I think after 1995. '98-99 Rivieras didn't have many options beyond the wheels and On-Star package. Not sure if the term was officially used by GM/Buick, or if the dealers coined the phrase, but it is acceptable to call the chrome wheels "Premium" or "Signature" series rims.
Adriatic blue was an interior color available with white exterior paint. I don't know if navy is the correct term, but it is a medium/dark-ish blue. There were many color options for interior, and may have been a darker blue, but I've not seen it. There where no soft tops for 8th gen Rivieras from the factory - this may have been a dealership ad-on, or a custom option from a previous owner. Same goes for the gold emblems and accents - these were not original options. Many cars are resold with gold embellishments as a dealer add-on package (sometimes called "24k"). Sometimes these look pretty good, especially on white exterior paint.
To post pics, click on the "host and image" at top of the the reply field, or set-up an account at Photobucket and copy/paste the URL info using the "insert an image" button. Best of luck!
'98 SC Riviera • 268k miles • 298 HP/370 TQ • 0-60: 5.79s • ET: 13.97 @ 99.28 • 4087 lb • 20.1 avg MPG • Nelson Ledges Lap: 1:30
3.4" pulley • AL104 plugs • 180º t-stat • FWI w/K&N • 1.9:1 rockers • OR pushrods • LS6 valve springs • SLP headers • ZZP fuel rails
KYB GR2 struts • MaxAir shocks • Addco sway bars • UMI bushings • GM STB • Enkei 18" EV5s w/ Dunlop DZ101s • F-body calipers
EBC bluestuff/Hawk HP plus • SS lines • Brembo slotted discs • DHP tuned • Aeroforce • Hidden Hitch
'05 GTO • 49k miles • 0-60: 4.8s • 16.9 avg MPG • Nelson Ledges Lap: 1:26