It’s the natural variation already present in native plants which allows plant breeders to offer cultivars, selections, and varieties to the home gardener.
When I consider purchasing plants billed as ‘native’ I narrow my choices to selections of species naturally occurring in my area. Generally, a selection is propagated by selecting plants grown from ‘wild’ or ‘native’ populations. The plants displaying the desired trait are allowed to reproduce until a relatively uniform population results. These desired characteristics may be height, growth form, or some other general characteristic, but the chances are pretty good they have retained the characteristics which I need in my garden, such as drought tolerance, ability to grow in poor soil, or the ability to survive Wyoming winters.
Various color options don’t necessarily require hybridization (the intentional crossing of different species, usually with some human assistance). The following photos are of a naturally occurring population of Delphiniums in central Wyoming. The color most commonly seen is the deep purple, but here we have stark white also present along with a transitional pale blue.
The genetic variation is already built in. This is an example of phenotypic variation, meaning the variation is being expressed.
When planning your landscape and choosing plants, consider how cultivated varieties of native plants can differ from the species found naturally in the wild. There’s an interesting article posted at Native Plant Wildlife Garden that lists some reasons ‘cultivars’ may not be suitable to your application, even though they are derived from native stock.