My choice for mat is consistently a very pale cream; only the slightest hint of a warm tone. [I'm not great at describing colours]. It's warmer than bright white, but not even close to buttery. To me, the very slight warmth balances or complements the cool, neutral tonality of a fiber print.
A friend made a fiber print in my darkroom, and chose to have it double-matted; black over white. He's quite happy with it. I'm not yet convinced.