Light Sussex feathers turning yellow

I have a light sussex hen whose feathers are slowly turning a yellow colour.  Not buff but a proper light yellow colour.  I mentioned this to my oh earlier and he said he had noticed it as well so its not just me imagining things.  She has had a slight crop problem and we kept her isolated without food and used liquid parafin but it didn't really work so our vet suggested we just let her out and see what happens.  That was last Thursday.  She seems to be eating etc but is sometimes a little distance from the others.  As she is not obviously suffering we are letting her enjoy herself.  She has always been prone to a large ish crop and perhaps that wasn't the problem.  Has anyone else had this change of feather colour in a light sussex?  I think she is moulting slightly or has been but will try and have a closer look later. 

Thanks for that D.  I think you are right and that is what is happening.  They look yellow against the new feathers!

I have 2 light sussex hens - One yellowing with a bald back - but very slow in the moult, the other is a pure white, bundle of fluffy soft feathers who looks twice the size of the other!  Hopefully here day will come -