A lot of folks just plant randomly as space allows, and a colorful cacophony is always a joyful sight. But that being said, I'm OCD about color harmony (I'm a floral designer by trade- its ingrained in me now LOL). I find browns, beige and grayish tones, and muted colors don't look their best when near bright, clear colors (it can make them look dirty or mud splattered). Last summer I had a purple and white plicata next to a light brown/beige one and a gold and rust bicolor that had to be moved- it was almost nauseating to see them together!
Pastels look beautiful combined together, and earthy colors look good together. I consider reds kind of earthy in tone.
