You know what makes me chuckle when this subject comes around again?
You always get someone saying you shouldn't put stickers on your helmet.
The bloody things are covered with them when you buy them in the first place. ACU stickers, size stickers, BS stickers, manufacturers' graphics, the list is near endless.
Or do they all use special, magic glue direct from the Pritt Stick fairy?

I don't think it is wise to paint a polycarb. helmet, (unless, of course, there have been significant, technological advances since the seventies) but today's modern composites should be perfectly ok.

Ask yourself another thing; Would race teams paint their riders' helmets if it compromised the performance?

I for one always take all the stickers off my helmets.
Just free advertising for the cheeky sods.
That goes for the poxy ACU gold sticker too.