The color of the 1857 3¢ Washington stamp was not "changed" as a result of a specific policy in 1857, but rather evolved through continuous production variations and a major plate redesign.
The primary reason for the distinct color difference between #25/#25A and #26/#26A is the use of different printing plates and ink batches:
Scott #25 (Rose): This was printed from the original Type I and Type II plates (Plates 4, 6, 7, and 8). These early 1857 perforated issues retained the "Rose" or "Rose Red" shades common to the imperforate stamps of the previous years. Scott #26 (Dull Red): This represents the Type III design, which was printed from a set of newly made plates (Plates 9 through 28) starting in late 1857. These new plates were designed to accommodate the new perforation technology better, and the ink used for these mass-produced runs shifted toward the characteristic Dull Red.
