Gourney is a fine place to start. Keep in mind that "color theory" is different, in different contexts - color for painting is a specific topic, that has a different focus on the pure theory, which is usually taken more by graphic designers than fine artists. You don't need to be an expert on theory to paint, but the basics absolutely will help, starting with the color wheel, and applying the concepts to mixing color, value, and controlling temperature. It's not something you should expect to master right away, either. Take your time, and try to absorb the beginning material before moving on. Theory is half the battle - seeing color is the other half.