The contrast of blue amongst green is an analogous contrast, making it pleasing and easy on the eyes.
One of the quickest ways I know to make a design statement in a garden, without having to try very hard, is to find the biggest, honkin’ blue ceramic pot you can find, and set it somewhere in your garden. Important: Don’t put it right in the middle, but rather place it off-centre; to the left or right, and toward the front or back of your garden space.
There’s something about blue—especially the intense, punchy blues of ceramic pots—that simply sings in green spaces. It makes sense: blue and green are next to each other on the colour wheel, they’re analogous colours, meaning they naturally go together, (linked by blue-green in the spectrum) yet there is just enough contrast to catch your eye: the kind of harmonious contrast that makes this trick foolproof.
Any blue pots tend to look significant in the garden, they bring along a natural design statement wherever you place them. But remember, off centre is best.