F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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I've been using krita for a while now to do pixel art on android, it works great, but you need to play with the setting to make it work properly.
Recently I found Drawpile, which is based on krita but optimized for android. https://f-droid.org/packages/net.drawpile/
pixaPencil ☞ https://f-droid.org/packages/com.therealbluepandabear.pixapencil/
ignore the "source code no longer available". It works fine as it is.