Several of the pictures I colored seems to have disappeared from the app. What happened and will they..

Check your settings - the cog at the top of the screen (right hand). You will see an option in one of the two tabs to say hide pictures. If you click on it again you should see them all again. Also check your feed - you should find it there at least - that's where you can delete the images if you want to (I have to, sadly, or I'd run out of memory).