First, for the bath salts themselves you will need:
Baking Soda (a natural moisturizer)
Essential Oil (for these I used Lavender, but you may choose any scent you enjoy--I generally use Peppermint if I'm making a foot soak)
When making a small batch for myself I use 1 cup of Epsom Salts, 1 Tablespoon of Baking Soda and 2-3 drops of Essential Oil.
I generally prefer to leave mine white, but a drop or two of food coloring can be used to color the salts, or mica pigments can be found online or in soap making shops if you prefer a more natural way of coloring yours.
Rather than Test tubes, maybe you would want to place your bath salts in decorative mason jars. These could be different sizes as well, and make quite a nice table centerpiece, mixed with jars of flowers or candies.
I also know that M&M's makes personalized candies that would work with this as well. You can get them in your wedding colors, or with images on them! Test tubes would again be a great container, as they fill up rather quickly to keep the overall cost down, but still look great. Or fill larger containers so guests can nibble during the reception.