I checked some of my photos of Goldcrests.
This is one from a couple of years ago.
And this is one I took a couple of days ago.
As you can see their feet are certainly yellow and that seems to extend up the legs a little way.
Both of the above were taken in spring but the next one was taken in winter and only the feet are yellow with the legs looking brown.
A lot of birds get brighter plumage during the breeding season so perhaps this applies to the feet and legs as well.
Thanks for that Tony - I thought it could be something to do with spring but couldn't find anything online!