There was a small group of Willow Warbler feeding alongside my local brook today and I  found it interesting to note the colour of the legs which in my experience are usually lighter in this species compared to the Chiffchaff that, again in my experience, are normally black.  Also the wings in the Willow Warbler extend well beyond the tail in the Willow and not so far in the Chiffchaff.  These two individuals show that there is a degree of variation in the leg colour.

Darker legs than the bird below but not black!

The feet are definitely lighter in this individual, look how far the closed wings extend.

This is a Chiffchaff from earlier in the month. Note the different coloured beak and black legs.