An overview for planting an interesting container
First, you'll want to choose colors that not only go well with the container that you've chosen, but with each other. Cool colors, warm colors, dramatic combinations or soothing ones - it's your choice. Just avoid color clashes.
Choose textures that blend and contrast well -- spiky against fern-y, bold against fragile. That way each individual plant will hold its own in the mix.
Shapes can make or break the arrangement. You don't want all rounded blobs, or all spire-shaped flowers but rather a pleasing mix of different shapes for variety.
Finally, watch proportion. A gigantic fountain grass will overwhelm a fragile forget-me-not and a giant sunflower surrounded by ground-hugging ivy will just look silly. Plants need to be in scale with each other to create a harmonious picture.