It's Possible but not Probable
Coticules vary from stone to stone. When I get a shipment of 20 kilos of belgians, I lay them all out together on a table under good light. I'm always amazed at how different they are one from the other and how different from those received in the past. The differences include color, shade, figure, and reflective differences. None of that affects the grading of the stone as I always buy Select grade stones which is the highest grade they sell. Looking at these stones under magnification is really eye-opening (so to speak). I recently purchased a video magnifier unit ($1,800 unit) - a monitor with a movable table and buttons for auto focusing, changing of light, background light, etc. and it's amazing what I see on ALL stones, not just the natural stones, and not just the coticules. The natural stones are natural products after all and they will have variation. Anyone who has bought diamonds knows how they vary! Same is true for other naturally grown products.
That the stone is mustard colored (would that be Guldens or Grey Poupon?) rather than custard colored (would that be flan or tapioca?) is irrelevant as to whether it's kosher or not. Kosher isn't dependent on the color of the stone at all!
Howard