code: public static double getDelta(ColorRgba a, ColorRgba b)
      {
          Check.notNull(a);
          Check.notNull(b);
  
          final int dr = a.getRed() - b.getRed();
          final int dg = a.getGreen() - b.getGreen();
          final int db = a.getBlue() - b.getBlue();
  
          return Math.sqrt(dr * dr + dg * dg + db * db);
      }
docs: Return the delta between two colors.
  
  @param a The first color (must not be <code>null</code>).
  @param b The second color (must not be <code>null</code>).
  @return The delta between the two colors.
  @throws LionEngineException If invalid arguments.