Unless it was designed that way, it is kind of hard to impose a color scheme on an established garden, especially if you have a spontaneous personality type that succumbs to the charms of any special-special plant seeming to speak to you and you alone at the plant nursery, and have to bring it home despite the fact that it doesn’t fit into your existing garden design.
If you do have the discipline and willpower to stick to the plan, some color themes are easier to maintain than others, because nature itself designed them that way. For instance, a white, yellow and purple theme will last indefinitely; those are the colors wild flowers come in, these hues are part of a packet of dominant traits which impart on the plants resilience and adaptability. White, yellow and purple perennials make for the most efficient color palette, they tend to live longer, need less maintenance and be healthier than the rest.
If your project is in the shade, go for all white. Most shade flowers tend to come in this color anyway. Purple is also common, but it doesn’t stand out as well from the foliage.
Sunny areas are always happy to support yellow and orange flowers, especially the late summer ones, which will keep blooming way into the end of fall, often even after the first couple of frosts.
Purple flowers are the most resilient, and I find my garden shifting to purple every year, no matter what I do. Their bloom tends to last longer than the the other flowers they share their blooming time with, so if you decide on a no fuss monochrome design, this color scheme would be the easiest to maintain.