Art Swirl creates abstract geometric line-based blends that are a bit reminiscent of string art from the 1970s, but don’t let that description discourage you, because I thoroughly enjoy the artsy effects of this App.
You can create manually, selecting colors and placement for the vertices which create gradations from line to line, or you can let the program create for you. I enjoy using it both ways. You also have several options like triangular, square and circular, as well as working against a black or white background.