Silver was first introduced as a new color in 1997 with Mega Silver. The label 'Silver' has been used on quite a few Rangers in Super Sentai, more in Super Sentai than Power Rangers. Each of them are different degrees of silver, I thought I'd cover them.
Mega Silver / Silver Ranger [Megaranger (1997) / Power Rangers in Space (1998)] is the first ever silver warrior, it was more like a white, a shimmery platinum-like color. In Power Rangers, it was much more bland and white-ish. The silver in
GaoSilver / Lunar Wolf Ranger [Gaoranger (2001) / Power Rangers Wild Force (2002)] was a solid silver, shiny but still grey-ish. Plus, it was accompanied with the color indigo.
Deka Bright / Nova Ranger [Dekaranger (2004) / Power Rangers S.P.D. (2005)], who was not officially called silver in neither incarnation, her degree of silver is indeed bright, brighter than Gao Silver but not brighter than Mega Silver. It is like a light grey, it is shinier than Mega Silver. Of course, Deka Bright's suit was half-navy.
Bouken Silver / Mercury Ranger [Boukenger (2006) / Power Rangers Operation Overdrive (2007)] is super shiny and metallic, almost alluinium. His suit also had black and orange, a bit of red and that shiny blue.
Go-On Silver / Ranger Silver [Go-Onger (2008) / Power Rangers RPM (2009)] is very shiny and aluminum-like and a bit blue-ish, almost the same as Bouken Silver, maybe Bouken Silver was more blue-ish, where Go-On Silver is the only one that is definitely metallic silver, not blue-ish, grey-ish or anything. Her costume is half black. Mega Silver, Gao Silver and Deka Bright all had some gold in their outfits.
I got a very polite request to cover what colors have been covered for the Sixth and Additional Rangers.
Green (3): Mask X-1 (1987), Dragonranger (1992) and Shurikenger (2002)
Silver (5): Mega Silver (1997), Gao Silver (2001), Deka Bright (2004), Bouken Silver (2006), Go-On Silver (2008).
White (4): Kibaranger (1993), Deka Break-with navy (2004), Deka Swan-with orange (2004), and MagiMother (2005). I'm not counting Geki Chopper because he is technically a fifth ranger.
Black (3): Kingranger (1995), BullBlack (1998), and Deka Master (2004).
Gold (4): Deka Gold (1994), Magi Shine-with navy (2005), Go-On Gold (2008) and Shinken Gold (2009).
Red (2): Time Fire (2000) and Aka Red (2007). Yes, I am counting Aka Red because where else where he go? And Wolzard Fire of course.
Purple (1): Wolzard (2005); let me remind everyone that GekiViolet is part of the main Gekiranger team.
They probably stuck most to metallic colors, I don't know how you would categorize the colors black, white, silver and gold, they are colors that are not primary and not secondary. Green and Purple are secondary colors, Green has only been used twice. Red has only used once in an entire series; AbareMax was red but he was only a power-up and is not considered a Sixth Ranger by fans nor officially anymore. The colors used by the female additional rangers had been used by male Rangers, except for Deka Swan, whose colors white and orange were later used in Geki Chopper.