Apps drawing in android and checking pixel color

[rajdey1]

I am working on an android drawing app and hence I want to know what will be the efficient way to check that none of pixel in the drawing area is left out white.(without color)

For your information I have created a class extending view and there implemented ontouch listener and overriding ondraw.

And whenever all the pixel are colored I would like to give a alert to user. Kindly help in this regard