Hi Cammie, welcome to HW! This is a great place.
I personally don't have mucus, but I know many people on this board do. If I am not mistaken the mucus is usually almost clear. I think it is anywhere from a light brown (tan) to clear. I'm sure others who experience this will post an answer for you.
I am so sorry to hear about your loss. Sometimes emotional stress can wreak havoc on our bodies. Are you having any urgency and/or diarrhea with the mucus? Any pain or cramping? These are all classic IBS symptoms (gas too, which you already mentioned).
So you are having a sigmoidoscopy in about a week? Has your doc said why... does he suspect anything other than IBS? The procedure is common for ruling out IBS, but I just want to know if he may have thrown out any other ideas to you (like Crohn's, Celiac, UC, etc.).
Again, welcome to the board!