In a garden of all neutral, pale, and green, almost anything of color will stand out and most probably appear decorative. Here we have a door and a window frame painted blue that can't help but stand out and be noticed. And among all the basic bland tones, it can be considered decorative.
This door and window frame are just a representation of what color can do in the garden or landscape design. A blue pot, red table, or any garden decor item of almost any color would stand out in a garden or landscape of mostly neutral.