To me, these two 24˘ stamps from the 1861 series are among the hardest to distinguish. Why? Because the answer lies in the color alone. No design differences. No hard or soft papers to compare. Color alone separates these two catalog numbers.
Scott #70 is listed in the Scott catalog as red lilac. Scott #78 is listed as lilac. It’s a piece of cake, right? And to complicate matters, both stamps come in several shades that merit minor catalog listings in the Scott US Specialized catalog. Further complicating matters is that color changlings are possible on this issue due to natural causes (exposure to sunlight, fading, etc) or artificially induced (chemical changlings).
Scott #70 comes in several shades: #70a (brown lilac), #70b (steel blue), #70c (violet), and #70d (pale gray violet).
Scott #78 comes in several shades too: #78a (grayish lilac), #78b (gray), and #78c (blackish violet).
How do you tell these two stamps apart? To me, it is difficult and I handle many of these stamps. To me, the #70 is slightly brighter (in general) and has a hint of more red color to me. It is subtle. To me, the #78 is more of a darker gray color with little or no traces of red color to the ink.
One way to help distinguish them is to use the “Encyclopedia of the Colors of the United States Postage Stamps, Volume I-II” by Roy White. However, a copy of this book is a few hundred dollars and cost prohibitive to most collectors. However, I find the color illustrations very helpful.
Another way is to buy certified copies of these stamps to use for comparison. Sometimes, a collector submits a stamp thinking that it is one of the rare shades and the certificate comes back with a normal shade. Buying an authenticated stamp such as this can help distinguish ink colors.
I wish that I could give you a definitive answer on how to distinguish these stamps But there is no foolproof method.Sorry! The only way is to study known examples of certain colors of ink and compare the copy in question.
On the scarce/rare shades, a certificate of authenticity is important because it’s easy to try and fake one of these shades.
Good luck in trying to distinguish these two Scott numbers!