Unless it was designed that way, it is kind of hard to impose a color scheme on an established garden, especially if you have the type of spontaneous personality that succumbs to the charms of that special-special plant seeming to speak to you and you alone at the plant nursery, and bring it home with no regard to the fact that it doesn't fit into your existing garden scheme.
If you do have the discipline and will power 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, since those are the hues many wild flowers come in, the most efficient color palette.
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 for shade plants, but it doesn't stand out as well against the dark 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 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. They tend to last longer than the the other flowers they share their blooming time with, so if you want a no fuss monochrome design, a purple color scheme would be the easiest to maintain.
The garden was delightfully fragrant this summer, and not from the usual sources, like roses and lilies, but from the faithful, easy to care plants, the hostas, the garden phlox and the petunias.
I always try to find new plants to add to the garden for scent; some thrive and some don't come back the following year, but even plants well known for their fragrance are sometimes reluctant to release it. I've been looking for ways to enhance the plants' scent, but I couldn't find any advice on gardening practices that would make it so, and therefore I'll have to rely on the garden to surprise me.
There are things that make flowers' scents stronger, just none within the gardener's control. Flowers release fragrance to attract pollinators, which are the only reason plants bother with scent in the first place, and they do so at the time of day when the particular insects that are attracted to their flowers are most active.
As far as weather conditions go, the scent stronger in the afternoon and evening, especially during hot and humid summer days.
Scent takes a lot of energy out of the plants, which is not something we often think about, and for some hybrids, the scent had been bred out of the variety to extend blooming time and vase life.
Some plants are said to increase the flavor of vegetables, and I'm going to go out on a limb and say that they probably enhance scents too. Onions, for instance, supposedly stimulate the plants around them to release more fragrance, and they are not such an outlandish idea for the flower bed, especially the large flowering ones, which are very close relatives of the drumstick alliums and whose white and purple ball flowers can look stunning in a mixed bed of perennials.
Scented foliage brings the fragrance with a lot less effort on the part of the plant, so don't forget to plant scented pelargoniums, lemon balm, mint and bee balm. Licorice hyssop's fragrance is surprisingly strong.
This last summer staple reminds me there is one thing the gardener can do to enjoy more scent: place fragrant plants along the garden path, where they will release their scent every time you brush against them.