The term "double-colored buti" likely refers to a pattern on the envelope. "Buti" generally refers to a small ornamental motif or pattern used in traditional Indian textiles and designs. In the context of a shagun envelope, a double-colored buti could mean that the pattern or motif is created using two distinct colors.
These envelopes are often crafted with vibrant and rich colors, intricate designs, and sometimes embellishments like sequins, beads, or embroidery to make them visually appealing and suitable for celebratory events.