Edit: I decided to go with materials instead of colors.
1 Year Member - Wood
2 Year Member - Stone
3 Year Member - Corroded Bronze
4 Year Member - Slightly Rusted Iron
5 Year Member - Shining Steel
6 Year Member - Polished Copper
7 Year Member - White Alabaster
8 Year Member - Shiny Silver
9 Year Member - Rosa Egeo Marble
10 Year Member - Glittering Gold
12 Year Member - Spessartine (orange) Garnets on Glittering Gold
15 Year Member - Rubies on Glittering Gold
20 Year Member - Sapphires on Glittering Gold
25 Year Member - Emeralds on Glittering Gold