b-y m1  ->  [Fe]
Hilker, 2000, A&A 355, 994
Revised Strömgren metallicity calibration for red giants.

m0 = m1 + 0.30*(b-y)

Valid for:
  0.5 < (b-y)0 < 1.10 
  0.1 < m0 < 0.90

[Fe/H] = ( m0 - 1.277[50]*(b-y)0 + 0.331[35] )
           /  ( 0.324[35]*(b-y)0 - 0.032[25] )

mean error = 0.25dex