CLIP STUDIO PAINT is a drawing app made by Celsys. In Japan it is often called "Kurisuta." It is made for drawing illustrations, manga, and animation, and it has many brushes that look like real art tools.
It runs on Windows and macOS computers, iPad, iPhone, and Android. There are two versions: PRO for illustration and EX with more features. A free trial is available.
For coloring, the fill tool colors an area inside lines in one step. It can fill areas even when the lines have small gaps. It can also keep color from going outside the lines, and it supports layers.