There are brush and pen types available in the "Concepts" , "Painter" and "Huion Sketch" app on my Samsung tablet that give the results in the attached image. You start at point A and follow a curved left to right path and end at point B. The curved loops either side of the AB axis are automatically filled with color. Some apps call it a fill pen/brush another calls it a leaf brush. I actually though I came across this function in Designer or Photo when I was experimenting with them when I first got them 6 months. I must have been mistaken because I can't find it now using various search methods. Is a similar tool available in Affinity?