Well it's an intresting problem
I think the best way should be using an Entity with "LeftClick" and "LeftRelease" events. Also "MouseLeave" should be useful to check if the mouse exit the drawing area (or you could block the pointer to exiting the drawing area checking pointer coordinates).
So if "LeftClick" on the entity you should place something to simulate the drawing under the pointer coordinates. Maybe a non interactive entity like a little circle with the colour you want (obviously only the first time and after only if the mouse pointer has moved)?
Check if the pattern match what drowned should be a bit more difficult. Maybe you could check the "non interactive entities like a little little circle" coordinates? Uhm i don't know...