Hi Vicky,
I'm not sure if I can answer this one but I have tried to find answers as it is interesting.
The only reference about this problem in any of my books I found in Chris Stormer's book 'Language of the Feet'.
In the section about colouring and changing moods of the feet she says "Colour variations as the feet relax, tense or stretch reveal the effect of deeply suppressed emotions under duress".
For green she says "Extreme envy, bitter, discontent and dissatisfied. Look for the good!"
Well, I don't think she means dissatisfaction with your treatment which, I am sure, was excellent. Rather, that the client has some deep-seated issues which are manifesting themselves in the feet.
The fact that you have noticed this is good as you can now talk to your client and try discreetly to find out if there may be any feelings such as those mentioned. As the treatments progress your client may be able to 'open up' and release these emotions. If they are deep seated then it probably won't happen very quickly.
Hope this helped. Let us know how you are getting on with this. I am very interested.
Maggie xx