According to the description, violet has an antibacterial effect, the green filter absorbs formaldehyde, which is a solvent used in paints and treated wood panels and is released into the indoor air over time. This filter would be recommended for new buildings and after renovations, for example. The grey HEPA filter is the standard dust filter.

The anti-formaldehyde filter is actually rather useless, because if there is a formaldehyde source in a room, then the values should be measured (the Dräger test is very reliable). Otherwise, the FOPH also recommends keeping the exposure in rooms as low as possible or removing the source, especially in bedrooms.

I had a case with formaldehyde in an Ikea PAX cupboard. Two identical wardrobes (different date of purchase) and different places of production by Ikea. Ikea said that they no longer use formaldehyde in the wood panels, but my tests said otherwise. The Mi filter wouldn't have helped much either, because formaldehyde sources usually release it into the air over a very long time (months/years).

So not all of them are for all devices, there are differences too

An expert should be able to explain this precisely. Unfortunately, I am not! I usually order the grey Hepa filter because my spouse has a pollen allergy and the air purifier helps considerably in making her feel better at home.