Window boxes grace the front of houses and may be the only gardening space urban dwellers have. If you live in a condo, or apartment building, place the boxes on the railing of the balcony if the window sills are inaccessible. The boxes may be formal or informal, match the house or be of contrasting colors. Window boxes in the sun should be a lighter color so they don't retain heat and increase the temperature of the soil within it. Window boxes dry out quickly in the summer sun. Check them every day.
Orange and yellow are happy colors and glow in the sunlight. Plant a box of ornamental peppers in oranges, yellows and reds. The peppers are smaller than the edible variety and are long and thin or oval and fat. They grow to 12 to 18 inches high. Sweet potato vines are a vivid lime green and grow profusely. Propagate new vines by cutting off a stem and placing in water. Dwarf black-eyed Susan has daisy-shaped flowers and stands up to the summer heat. Finish the box with apricot calibrochoa, it looks like a petunia the size of a half dollar. Calibrochoa don't need dead heading like petunias do.
If purple is your favorite color combine various shades of flowers that bloom in purples from dark to light. Add a pop of white alyssum to set off all that purple. Good choices for a sunny purple window box are verbena. It trails down the sides of the box with tiny flowers grouped in circles. Petunias come in various purples from light to almost black. The plant grows to 18 inches high. If it stops blooming, cut it back to 6 inches. It most likely will start to blossom again. Alyssum has tiny white flowers that almost cover the 8 inch high plant. It grows to 12 inches wide and will soften the edges of the window box.
Summer colors are hot. Use flowers in gold, red and oranges. Salvia has stalks of bright red flowers that grow to 12 inches high. Contrast them with red pom pom zinnias, orange and yellow petite marigolds and coreopsis. Lantana is a perennial in warm winter areas and used as an annual elsewhere. If the window box is large the lantana will grow quickly to fill it. Lantana needs light pruning to stay bushy and keep on blooming. It blooms in various color combinations of reds, pinks, yellows, oranges and purple, often on the same plant.