Once again, it's relatively simple. Choose your favorite color or any color for that matter, and use any colors you want along with it as long as they share a common color. For example, a bed full of white, pale pink, bright pink, and red begonias is very harmonious because it consist of colors all made from red and white. These intermediate colors are made from red and white together. Theoretically, you can tie any colors together by using intermediate colors between them.
Nature is a very good teacher here.
Take a peach for example. It's really yellow and pinkish-red. But look between those colors and you see a multitude of yellow, pink, and red combinations. Stay on the lookout for flowers which are a mixture of colors. They allow you more flexibility in color scheme choices.