Colors: Brass-based Centuries were in production from the very late 1960s (before that, they were pale copper) until 2003. The brass Century with the screw-on cap is the most common of all Lava Lites. Uncommon colors are ORANGE/yellow (switched to blood-red by the early 1970s) and GREEN/blue (switched to cream, same time). Copper-finished bases are very uncommon-- they have an inner cylinder attached to the top half of the base, which will slide part-way out if lifted.
Aristocrats are slightly less common-- Midnight Aristocrats are MUCH less common, especially with screw-on caps. The black Aristocrats were first introduced as a Target-only model in yellow/blue or yellow/purple. The rarest are the high-waist Aristocrats from the early 70s (with a taller, narrower upper cone) and the Commander, a short-lived early-70s matte-black version.
Silver appeared in 1997, then black, and both had screw-on caps for a bit before 52oz. lamps transitioned to crimped caps. These were also produced, as far as I know, until 2003. Both are uncommon, though more so with screw-on caps.
In 1997, screw-cap silver came in red/yellow, white/blue, pink/pink, red/clear, blue/clear, purple/clear and black/clear. Screw-cap black came in these plus pink/clear and dark green/clear. Screw-cap, 52oz. pink/pink or white/pink, red/yellow, and blue/clear seem to me to be less common.
Crimped-cap black, silver and brass Centuries aren't rare, though I think brass ones with crimped caps are less common than black or silver.
The truly rare Centuries are models like the ceramic-base type (one known to exist), the early models with a heat sensor in the base (zero known to exist), and the very early models with no seam in the base's pinch, a switch on the base, and liquid-when-cold lava (possibly one known to exist).
Jonas - thanks for all this info. You answered a lot of questions I had as well.
Glad to put it out there.