Hi @gim
A great question!
There is a difference between the two filters past that of the colour.
The Post Filter captures 99.99% of particles down to 0.3 microns. This includes the likes of dust mites, pollen particles, mould spores, skin flakes and those particles the size of some bacteria.
The HEPA post filter captures 99.97% of particles as small as 0.1 microns. Further trapping ultra fine particles such as pet dander and diesel soot. Helping towards creating a cleaner and more hygienic home. This additional filtration allows for the naming specification as HEPA.
This HEPA filter is available for purchase separately and included with certain variations of the Dyson V15 range.
I hope this helps.
Stuart