Different polymorphs of the same compound may have different colors, but when dissolved in hydrochloric acid - their solutions will show the same color.
Iron with a concentration above 1 ppm can be determined colorimetrically, qualitatively even visually, roughly quantitatively - by comparison with the color scale.
Higher mass after drying results rather from shorter time or lower drying temperature (amorphous aluminum hydroxide may need more time for drying).
100 ppm Fe3+ in HCl is rather colorless - it can be easily checked in the lab.