GetPalette.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using PicaVoxel;

public class GetPalette : MonoBehaviour 
{
	void Start () 
	{
		// Initialise a list of colours to hold our palette
		List<Color> palette = new List<Color>();

		// Get a reference to the voxel array
		var voxels = GetComponent<Volume>().Frames[0].Voxels;

		for (int i = 0; i < voxels.Length; i++)
			// Only include the colour of active voxels, and check we haven't already added it
			if (voxels[i].State != VoxelState.Inactive && !palette.Contains(voxels[i].Color))
				palette.Add(voxels[i].Color);
	}
}