I use a silicone sheet when I use starch. That way I don't have to keep
cleaning my iron.

I've had that happen. In my case, it's the starch that turns brown, not my fabric. What works best for me is to spray the starch, then spray a little water on top and wait about 30-60 seconds. This seems to help the starch settle into the fabric, so it doesn't burn and turn brown when the iron hits it.

I've had some experience with starching. I keep a piece of muslin to lay over the ironing board for when I starch. It catches the spray and helps keep starch off the ironing board cover. Also, I use a mesh heat resistant cover:
https://www.walmart.com/ip/High-Temp...olor/553644369
Usually I'll starch my fabric while still damp from being prewashed, and I'll turn the fabric wrong side up on the ironing board to iron. Then I have to be very careful and use a light hand while ironing. Seems like starched fabric can more easily become stretched out of shape. If the iron is too hot it will burn the starch causing scorch marks, which will wash out. Sometimes the burn will go through to the fabric; I had that happen once. Hoping you just have burned starch and it comes right off.

I hang large pieces of yardage over the shower curtain. Spray with starch and let dry.

The other option, is to sub cut the yardage before starching. There may be a little fabric waste. It is worth it to me to be able to handle smaller pieces.

I add an inch on each side of the piece to allow for shrinkage and to clean up the cut edges. I don’t need a 3 yard piece of fabric on the ironing board or cutting table. It’s much easier to work with the smaller pieces. Good luck!