Catalyzes the S-adenosyl methionine-dependent conversion of 8-amino-8-demethyl-D-riboflavin (AF) into 8-methylamino-8-demethyl-D-riboflavin (MAF) and roseoflavin (RoF), the last two steps in the biosynthesis of the antibiotic roseoflavin. 8-amino-8-demethylriboflavin + 2 S-adenosyl-L-methionine = 2 H(+) + roseoflavin + 2 S-adenosyl-L-homocysteine kcat is 22.0 min(-1) with AF as substrate. kcat is 14.8 min(-1) with S-adenosyl-L-methionine as substrate. Optimum pH is 8.8. Optimum temperature is 52 degrees Celsius. Antibiotic biosynthesis. Homodimer. Inactivation of the gene leads to a strain that synthesizes AF but not MAF or RoF. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.