I'm currently working on an idea for a Unity game app with a canvas mechanic. I understand I may have to use particles for the brush/paint f/x however I can not think of a way to add collision detection let alone the paint itself...onto a canvas... Any suggestions are much appreciated!
You can use both particles (for the animation) and another system to really "paint" the surface. What I would do is use one or multiple invisible spheres that I would "trow" in direction of the canvas (with a Rigidbody, so it's affected by gravity), and then, detect collisions.
You could also use Raycasting.