Sometimes your subject offers you all the color you could need, and your challenge becomes to represent those in a unified manner, by modifying some colors, omitting others... And sometimes, the colors in the subject are only your departing point, that's when most of the fun begins!
"Farm on Nelson Road" was done on-site not far from my house. I'd often driven by this scene at sunset and marveled at the gold of the field. This one weekend, I got on my bike and rode to the place, sat on the lawn across the street to paint it, but the colors were not as intense that day (see the photo on the left)... No problem, I visualized them all the same. I intensified the colors I could see, and added some colors of my own, often complementaries, as the orange in the roof, the touches of red among the grasses, the blues in the shadows (see detail below). My thinking process at that point was not "what colors are or are not there" but "what new colors would make the existing colors sing." This was a fun study to make, and my wife has a special affection for it.