That's a big question and requires a lot more information to answer. Need to know what plants you have already, which USDA zone you're in, whether you're talking about shady or sunny areas, and some photos wouldn't come amiss either...

Otherwise, there are quick ways to inject colour - people with low maintenance gardens which mostly contain shrubs and a few perennials often plant up pots with summer bedding plants, sometimes actually planting them in borders too, though this does make it slightly less low maintenance. Summer bedding only lasts the season but gives plenty of flower colour whilst its present.

Agree with Bamboo. Raised potted plants on well anchored plant stands to make them more visible such as impatiens, New Guinea impatiens, petunias, begonias(the fire begonias are spectacular), gerainiums etc. Also try the ever flowering tropicals such as Mandavilla vine and Hybiscus and the towering Canna. Consider all potteds to be annuals and read the growth requirements on the labels so you can place these plants in the proper locale in your garden. This method is a bit more costly though you will find discounters that sell most at half the price and potteds are a more continual care plant in the heat of summer but you will have your areas of color in your garden that you desire, from late spring to early fall and all you need are just a few well placed eye catching focal points of color to get the job done. With the end of the growing season now at hand you have plenty of time to design your garden for color next season.