There are many possible reasons what would makes PVA solution turns yellow:
1.Please check whether other material (except water) added (or mixed unexpected) or not. If other material added, please check whether those materials would make the solution turn yellow or raise chemical reaction with PVA
2.Please check the color of PVA solid, whether PVA solid has turned yellow or mixed with yellow foreign matters or not.
3.Please check the water you used. If the water contains high Fe+ or rusty water pipes would make PVA solution turns yellow
4.Make lab test. Use clean cup, clean water, make high concentration solution and check the color.
5.If PVA solid and lab test seems good, the possible reason is the reactor is rusted. Some rust can be found easily, some rust is not obvious. PVA solution is usually high  viscous, what would grab out all possible rust.
The reactor need be fully cleaned. Or produce with wasted material firstly, then use good PVA material to produce.
6.High temperature & long time heating normally won’t make PVA solution turns yellow, unless the water is completely evaporated