whites always a difficult shade to keep looking original, but there are cleaners available for white leathers, but your PVC/patent is not so easy!
the pigment in the boots changes over time, which is unavoidable
Prevention is better than cure here!
store them in a cool dry place, to much heat (near a radiator) will shorten the life of the upper.
Store them out of site of natural light, as uv light will accelerate discoloration.
BUT MOST IMPORTANTLY Following each wear leave the shoes/boots in a well ventilated area to allow the moisture in them to evaporate (your sweat contains acids which are not leathers best friends) this moisture will often soak through the leather/PVC/patent causing discoloration from the
inside out, which is not good in the case of PVC/patent.
Lee