colorType(number) {
    switch (number) {
      case 0: return 'grayscale';
      case 2: return 'rgb';
      case 3: return 'palette';
      case 4: return 'grayscale-alpha';
      case 6: return 'rgb-alpha';
    }
  }
Retrieves the PNG's color type.

@param {number} number A number representing the color type.
@return {string} A string naming the color type.