Current game stage: 1
Alpha 17 painting has changed and been streamlined. No longer are there paint texture schematics to find and unlock. Instead, simply crafting a paint brush (and with paint to actually paint) allows access to all the available textures.
While this might seem convenient, it also comes with a loss of a fair number of textures that were available in Alpha 16, but removed in Alpha 17.
Hold "R" to bring up the paint brush selection wheel.
Hit "R" to open up the texture selection palette where the textures can be reviewed and selected for painting.