Might be bucking the trend here, but as a wise man one said,
"It's your goat. Dress it up how you want."
This guy has the right to paint his AK any color he wants just like anyone has the right to put twelve pounds of unnecessary gadgets and accessories on a shotgun. We may not agree with or understand it, but that doesn't matter one bit.
What does matter is whether or not the firearm is being used responsibly. We had a case here in TN, Embody vs. Ward, where a local guy with a reputation for pushing the boundaries of firearms laws decided to take a walk in a State park with while open carrying an AK pistol. A newly passes law had allowed TN residents to carry in State parks, so he decided to push his luck on a little understood technicality. Embody painted only the flash hider of his AK pistol bright orange so it would appear as a toy. Park patrons panicked and called the cops. He was forcibly detained, disarmed, and arrested by a park ranger. The legal battle that ensued resulted in Embody losing his right to carry and set a legal precedent that one can lose their rights if their actions cause civil unrest or generally just scare people.
In the instance of the OP this guy was at a gun range, apparently using his weapon responsibly. He was not out to intentionally cause anyone harm or even provoke a reaction of any kind other that perhaps someone asking why the hell he painted his AK like that.
In my opinion, it's his gun, his choice. We cannot begin to decide his choices are wrong without opening the door for our own choices to be scrutinized. If we begin trying to impose our will on others simply because we don't agree with or understabd it, then we are no better than the gun grabbers trying to revoke our rights and limit our freedom to make our own choices...