Each material consists of a couple of paint layers to achieve the desired effect. Each of these mixtures can be mixed with 4 individual paints in any order or ratio.Tap the + sign in the upper right to add a new mixture. You are automatically pushed to the mix view of the new mix. On this view you can set all paints that make this mixture by tapping one slot besides the long mix bar. You can now choose a paint and be pushed back to the mix view. The stepper in this slot can now be adjusted to the ratio you want it to be. The mix bar will show you the ratio in a visual manner. You can change the order of the mixes slots by pressing the "Edit" button in the upper right corner and grab the slot by its reorder control on the right side.