It has always been my feeling that correspondences are terribly useful things when designing a spell. Utilizing the powers outside yourself to align them with your goal helps in achieving your ritual purpose. It adds to the strength you have individually by tapping into the archetypes of the universal unconcious. Particularly when you consider that many of these correspondences have been used for centuries, or even millenia. Imagine the power stored in their connections to ritual purposes! In addition, correspondences that you can feel are correct add to your own concentration on your ritual purpose. They focus your energies on the goal. I have always used as many correspondences as possible when designing my spells, personally, they crystalize my intent and assist me in raising the energy necessary for a successful ritual.
What follows are a number of tables for correspondences of all kinds; color, planetary, day, goddess/god, candles, herbs; a variety of items and from a variety of systems of magic. You must examine the tables and find the correspondences that resonate for you personally, there are many correspondences listed for each category of ritual intent, use the ones that feel right for you. That is where their real power lies, in the vibrations they set up in the environment during the ritual. If indigo has been used for centuries for a particular purpose it adds that vibe to your ritual for example. But it is important that you feel the connection between the correspondence and what it is supposed to symbolise. If you don't it won't be as effective. Remember, you are the magic!
Moon Phase Correspondences