Zen Brush

Zen Brush for Android is a simple but elegant app for simulating sumi ink brush painting. It has three brushes with different opacities ranging from dark to light. There is a size control for your brush and a soft and hard eraser. You are limited to one undo or redo. There are 62 different backgrounds that you can choose from. Some of the backgrounds have different ink colors such as brown, orange, green, purple, pink, blue, or yellow, but certain ink colors only go with certain backgrounds. You can't pick different colors of inks. 
There is a very limited free version of this app where you can try it out and see if you like it. The full version is $2.99 and unlocks all the functions and backgrounds. This app is developed by PSOFT Mobile and is also available on IOS.

Here is the UI. The Zen Brush app only has portrait mode. It is fairly simple with a brush size slider, three brush opacities, soft and hard erasers, undo button, menu, and delete button. 

When you select the menu you can choose different backgrounds through the 'Select Template' options.

There are 62 background selections and they are based on different types of papers and surfaces. For example, in this group you can choose Japanese paper or bright colored papers. The font color for the paper name will be the color of the ink.

In this group there are some patterned backgrounds and you can see by the font color for Spring 4 that the ink will be a light purple.

There are a bunch of different textured backgrounds such as wood and metal. 

They even have blackboard or cartoon backgrounds.

Here are the 3 different opacities for the ink. 

'Old Tree Stump '

For this drawing I used the Coffee Filter background.  The ink that goes with it is brown. This app is nice for making monotone type drawings and you can use the different opacities for shading. It is sort of like working in charcoal for me since I can't use different colors. Overall this is a fun little app and you can use it to get a nice sumi ink look. It would be nice to have a few more colors for the inks.