The yellow water doesn't always mean it's dirty. Betta fish are tropical fish that often live in black water biotypes. Black water is full of vitamins and minerals that will often promote good health in tropical fish and also usually contains peat extract, which is known to help make water bit soft and acidic (ideal betta fish conditions). It does give the water a yellowish tinge.
I use black water extract in my tanks (since I can't find indian almond leaves or anything else of the sort that would create the same water conditions) and I've noticed in my female tank that before I use it, their colours aren't as vibrant.
So where ever you got them from might've used something similar.