Wikipedia
Magnetochromism
Magnetochromism is the term applied when a chemical compound changes colour under the influence of a magnetic field. In particular the magneto-optical effects exhibited by complex mixed metal compounds are called magnetochromic when they occur in the visible region of the spectrum. Examples include K2V3O8, lithium purple bronze Li0.9Mo6O17, and related mixed oxides. Reported magnetochromic compounds are multiferroic manganese tungsten oxide and multiferroic bismuth ferrite.