Viola obliqua is a native violet and is always such a surprise in spring.  I should be used to it by now, but its bright white continues to startle and delight.  It is pure, pristine white with no veining, although the center often tends to be the palest yellow.  While it does seed heavily, I haven't found it to be a problem.  Besides, it's much prettier than any grass I've ever seen. (Photo by Björn S..., CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons)