I worked in the motocross industry for awhile. We always used water slide decals for the base of the helmet graphics. But you can spec over painting type techniques. Such as small amounts of airbrushing to add gradients. Also they can do small amounts of pin stripping to. Usually with a tape pinstripe material. Then heavy clear over the whole helmet. In my experience helmet manufactures are some of the best at in interpreting 2d art into 3d applications. This has just being my experience I am sure it is different for each factory. Water slide will still have some amount of distortion in areas. But the factories will generally work with you to correct these issues.