Around our house, this is a very literal thing. Brightly colored foods are more appealing to a preschooler with a poor appetite than bland looking food. Rainbow anything is a win!
Red; Orange; Yellow; Green; Blue; Indigo; Violet
One recent experiment was rainbow pancakes. Using various foods (strawberries, mango/papaya, pineapple, spinach, blueberries, grape preserves), we colored the pancakes before tasting what we'd made. It was a fun experiement and helping in the preparation helped Lily had a slightly better appetite. (FYI: Spinach pancakes aren't so good. Gotta work on that!)
With a small child that doesn't eat well, when she does eat, I have to make every calorie count. I have to try to squeeze in everything that she should be eating during the day from the various food groups.
Applesauce milk with oatmeal baby cereal... It doesn't sound very tasty, but 4 ounces of pureed apple, a couple of tablespoons of single-grain baby cereal, and 8 ounces of milk. She's got a "milkshake/smoothie" that's got a serving of dairy and a serving of fruit, with a little bit of grains and fiber.
For me, orange is my food color right now. I'm all about oranges and grapefruit right now. Of all of the fruits, they're the ones that are usually on sale. Not to mention with cold and flu season in full swing, they pack some mega vitamin C.
So, for us, right now "Eating Right with Color" has heavy emphasis on color.