The colour wheel
This is a tool I use every day. I bought it about a year ago because I'd seen other artist using them. I left it in my drawer until a couple of months ago when I started doing some research on how to use it. That has been such an eye opener, especially when it comes to using complimentary colours.
In relation to this topic I can strongly recommend the book Color and Light by James Gurney. Whatever style or technique your're using, this book helps you understand how colours work together.