I have a drawer full of watercolor swatches. These are palettes that I've used for previous paintings and/or just colors that I've mixed together and like. The colors in each swatch are noted in abbreviations because I tend to forget what I've used. These swatches are a great starting point for paintings. When I'm first considering the mood of an illustration, I'll often empty this drawer in front of me and start picking through for colors and combinations that give the right feel. Also, because this particular project is multi-page, it helped me to hang swatches/color palettes with the sketches (all over my walls) so I could see how the various color palettes work together.
A quick side note: I was the winner of a fabulous onesie from Joy Nevada's Etsy shop. Her illustrations are whimsical and suited perfectly to apparel for little ones. Thank you, Nevada!