I love to work with graphics and I never pass an opportunity to try a sketching app.
I like how you planned the interface, very clean and quick to understand, so people can focus more on what they need to do instead of learning some weird interface.
Also, I like the zooming feature, even though it doesn’t zoom in a lot, I guess you can control that and make it magnify the image a little more may be?
I noticed when I tried to exit the app that I had to press back a lot of times and the same screen kept showing up. You are probably opening the same screen a lot of times. Eventually that will make the app run out of memory and force close. Remember to open new screens and close those instead of opening screen1 again.
Another thing you can take a look at is the color and line width I select, doesn’t stay when I open the SETTINGS option. After I go back to the drawing screen all is reset to the default values. It should remember those, may be in a TinyDB?
But overall, congratulations on your sketching app. It’s very nice.