Sin Rummy uses the same base rules as Rummy, so if you usually play Rummy with only 1 deck of cards, then you only need 1 Sin Rummy deck.
Traditional Gin Rummy is 2-player and only uses 1 deck. Multiplayer versions of Rummy 500 use 2 decks once there are more than 4 players, and Canasta uses 2 decks - so if you want the option for 4 or more players, then 2 decks is probably best.
For this reason I gave the option of 2 colors. Personally I like having 2 around because then I can also use them for things like Bridge.