I'd guess it's just bicolored. This is very typical of irradiated pegmatitic topaz, especially from Pakistan. You can also sometimes see sector zoning and odd tigerstripes.
Look at how you're lighting it, though--the yellow zone is only on the side opposite to the light source. My guess would be that it's the c-axis color of the brown zone being flooded into the colorless rind. One way or another, be sure to keep it out of the sun!
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