Since Marshmello appeared in the music scene back in 2015 we've seen him go through multiple helmet changes and upgrades over the years, but the one that stood out to me the most was the helmet Ironhead Studios built for him.
So the first order of business? REFERENCE! I spent a bit of time gathering different pictures of the "Ironhead" helmet from the official Ironhead Studios/Marshmello Instagrams and many other places across the web.
After a bit of research I've come to the conclusion that Ironhead Studios made two different helmets for Mello. The first being a "static" helmet without the LED display inside for use at red carpet events or other public appearances and a "illuminated" helmet mainly for use on stage, although he has been seen wearing the the illuminated helmet around before.
Above we have what I call the "static helmet" this is the helmet Mello is seen wearing most of the time. This particular helmet has a white glossy finish and what appears to be a fine perforated mesh in both the eyes and mouth, I'd imagine this one is lighter and easier to wear due to not having the LED display and other electronics inside. I didn't include a picture of the underside of the static helmet but he does have a gasket made of some kind of material, most likely to further hide his identity, or just to cover his chin.
Now pictured above is what I call the "Illuminated helmet" Mello is seen wearing this one primarily on stage but has been spotted wearing it to a few red carpet events. What makes this helmet different (apart from the LED display) is the eyes seem to have lenses while the mouth retains the perforated mesh, probably so that his voice can still be heard through the helmet when he's using a mic. We can also see what looks like holes of some sort around the base of the helmet, most likely for ventilation as I can imagine it gets quite warm in there during shows... We can also see that gasket I was talking about earlier in the second picture.
There was a bit more research done but this is just the quick rundown of the two helmet variants.