It looks as though some of your potatoes might have got a bit scorched. Did you fry them off before adding the liquid? The colour from the browned potatoes could be what is turning it brown.
So I've made a pan of L&P soup today left it simmering for 25 mins only added a veg stock cube as i always do and when i took the lid off there it was brown!! This has never happened before its usually pale greenish white. It looks horrible as you can see. Any ideas as to why?