I recently re-discovered a painting app called PaintRack. It is free, but $2.99 unlocks some features. That said I think it is an invaluable tool. I use multiple brands of paint, and organize them into tetrads. This app allows me to organize and track my paints, but more importantly, I can track recipes. The one feature currently missing IMHO is the ability to record a custom mix paint (a confirmed future addition).
LIBRARY: track your paint inventory with over 6800 paints from over 25 manufacturers!
SETS: create custom paint sets or recipes
WISH LIST: track paints you want
RAPID SCAN: add pints by scanning the bar codes on bottles
COLOR TOOLS: color match with camera or within app; breaks down complimentary, analogous and triad colors by brand.
Here are a few of my screen shots showing my metallic tetrads, my wish list, and a recipe for a recent project.