Hi Andy,
I too had yellow staining which was light in colour and sometimes a bit gooey. At first it was brown (and occasionally pink) then once it started to heal it turned light yellow which was the wound weeping. I was at this stage for a little while and it eventually went away by itself.
Is this yellow coming from your behind or is it on the medicated gauze? If it is on the gauze and does not have a strong foul smell then that sounds normal.
If you are still concerned give the CRS practice nurse a call and she will cheerfully give you advice (you probably met her on surgery day), if she is concerned she would get Mr Windsor to call you.
Anyway I hope all else is well and that this is just one of them fearful blips we all get
Davo