Krita

Krita is a professional painting program.
It is made by artists that want to see affordable
art tools for everyone.

It's designed primarily for digital painting and
2D animation and features an OpenGL-accelerated canvas,
colour management support, an advanced brush engine,
non-destructive layers and masks, group-based layer
management, vector artwork support and switchable
customisation profiles.

Note: android version of krita is optimized only
for tablets and may behave poorly on phone screens.