If you now tap on the blue chunk adjacent to the empty corner voxel (any blue voxel will do actually) it will be removed.  After a few turns, you might end up here:


Clearly quite a few similarly-coloured voxels are surrounded by space, so at this point one tap can remove a lot of voxels.  Here we are one tap away from the end of the round:


Your score at the end of the game is the number of taps it took to clear the screen.  Try to get the lowest possible score!

If you have large fingers and cannot always tap accurately on the right voxel, there is an option to set a different selection mode in the Options view, available from the home screen.  Rather than tapping on a voxel to remove it, you can choose to have an aiming reticle in the centre of the screen and then tapping anywhere on the display will remove the voxel in the centre of the reticle.  It's a little bit slower to play, but the picking is much more accurate.

There is also an option to allow long-touch zooming.  If you tap and hold on the screen, after a moment the display will magnify several times which makes it easier to select the voxel you want.  Tap and hold again to shrink back to normal size.