The purpose of the plugin is to draw stuff to a canvas. You can already read pixels, but anything more such as keeping track of the object's that were drawn to it isn't what the plugin is for.
That said here is the color picking method:
https://dl.dropboxusercontent.com/u/542 ... cking.capx
Left drag draws a line and right click erases the line that was clicked on.
It works by adding the endpoints of each line to an array, then drawing the line on the main canvas as normal, and on a second canvas, only thicker and with a color that maps to an index in the array.
Then deleting a clicked on line is a simple matter of checking the color, deleting the array index that matches the color, and finally clearing the canvas' and redrawing all the lines left in the array.