so I assume that maybe stickers on an Alpha are permissible?
NO! But you CAN assume that the adhesive on the factory-affixed stickers , IS COMPATIBLE WITH THE SHELL MATERIAL.
Really, people, this could save your life (or kill you! )
Some helmet shells are machine moulded out of PolyCarbonate or ABS....They're both plastic, both will look the same, but their reaction, even to something as daft as spray-on furniture polish. could mean they split or shatter with a hard impact. Both plastics react differently to different chemicals......for "solvent" read"lots of commonly used household cleaners and polishes among other stuff" What may be safe on one, can be lethal on another....even sitting your helmet down on an oily hand -wiper could render it useless.
Then you have manually- moulded shells, Polyester or Epoxy resin, reinforced with glass /Carbon-Fibre / Kevlar.
Again, differences are pronounced.
In the latter days of the British motorcycle-industry's heyday, there was a vogue for "cafe--racers"....clip-on handlebars,rearset footrests and a big fibreglass petrol tank held on with a quick-release strap or bungee cords They were OUTLAWED when,in crashes, they disintegrated, tipping anything up to 6 gallons ofpetril across the accident scene.....an alloy tank will dent and leak a bit from the severed feed pipe, but won't form a giant molotov cocktail.
Stay safe! beware of using paint or stickers, the damage will be totally hidden until protection is required.