Colors are selected by using the Swatch Palette found by pressing the Material Button. Voxel Max contains several pages of preset colors, which can be seen by swiping left or right on the palette.
Tapping on a color will select it, indicated by a circle around the color. Tapping "Done" will close the palette. If a color is used in the file, its swatch will be indicated by a check mark.
Double tapping a swatch will open the Color Editor. Here you change the color using Hue, Saturation, and Brightness sliders, or tap on the left side to switch to Red, Green, Blue sliders. The circle on top will show the new color as a ring around the old color.
The bottom slider is the Alpha, or opacity value. Lower numbers mean the color will be more clear until it's invisible. To use transparent colors as glass, it's best to set them to a different Material Layer.
Tap the reset or “Back” buttons to undo your changes, or tap “Save” to apply the new color. Any voxel using that color swatch will automatically become the new color.
The six most recently used colors will appear above the palette button, so you can quickly toggle to those colors for faster painting.
Learn more about the features and tools in Voxel Max with this video.