According to my colon therapist, it's "ammonia". It shows that your system is cleaning itself out. I had the same thing today (I'm on day 7 as well). Do you feel a little sluggish or uncomfortable before it comes out? That happens to me. It can also feel like a burning sensation. When I have a colonic, I always know when that yellow stuff is coming out because I feel a little shaky and burny...but it's pretty much being forced out with the colonic, so the feelings are a little more extreme. No matter what, though, I'm always glad it's out.
You might also see mucus leaving your bowels, but you really have to look to see it.