Third Degree Assault (the mildest infraction) in Denver (and Colorado in general) is defined as:"knowingly or recklessly causing bodily injury to another person." (In rare cases, a different definition is used based on accidentally causing injury with a deadly weapon, but that definition is not relevant here since the act in not "accidental" and the weapon is not "deadly".) Regardless, notice that there has to be "injury" in order for the charge to be supported. "Pain," by itself, does constitute injury - even if there was no physical damage to the person assaulted. In some cases, "pain" defined as "injury" can include psychological pain, such as "fear of violence." (This "fear of violence" logic usually covers incidents where a perpetrator shows a weapon in order to persuade the victim to take part in undesired actions.) The maximum sentence for Third Degree Assault in Colorado is two years in the county jail.
So, by this relevant definition, it is technically possible to charge the person with Third Degree assault. The argument would be the following:
Did the glitter-bomber knowingly do the act? - - YES (relatively indisputable)
Did the action cause injury to the victim, such as psychological pain? - - YES (at least the victim is allowed to claim thusly)
So those are the only qualifications needed to charge the glitter-bomber.
However, being charged with something is way different than tried for something, let along being convicted.
But, by this bare-minimum qualification, one could charge a kid for shooting a spitball at another kid, even if the spitball misses... heck, even if the spitball is merely aimed at the kid, hence causing him "fear of violence." Since society generally does not want to take spitball-shooting kids to court, the legal system usually functions off of precedents. Courts look at prior cases to find a general principle or rule as to how any certain law is intended to be interpreted/enforced. As far as I know, no court has ever taken a spitball case to trial. Thus, if for some reason someone did manage to charge a kid with assault for shooting a spitball, the case would probably be thrown out very early in the process (in preliminary hearings) since the charge does not measure up to the accepted intention of the law.
By this principle, I contend that the glitter-bomb is akin to the spitball example. In fact, it can be argued that a potential spitball in the eye at high velocity is more eminently dangerous than falling glitter. Regardless, I would surmise that this charge will either be dropped by the complainant, or it will be dismissed in preliminary hearings. If it does happen to go to trial, I highly doubt that the court would convict this on assault. At worst, they would lower it to a different charge such as littering or some sort of negligence charge. The court would not want to convict on assault because that would set a new precedent as to what an assault conviction can be, basically expanding the accepted definition of assault to include many things that are merely nuisance behavior. An assault conviction on glitter-bombing would open up the court to a whole slew of trivial charges like spitballing, putting ice down people's shirts, pranks like writing on people while they sleep, pushing people in line, etc. Lawyers could then argue: "[stupid-thing-X] has been scientifically proven to cause more injury than glitter-bombing; glitter-bombing has been deemed as assault and therefore [stupid-thing-X] is also convictable assault." The courts do not have the time to deal with this garbage so they will avoid setting a precedent that allows for it.
Nonetheless, it is clear that the charge made on glitter-bombing is meant to send a message and make a public statement. This official charge got some headlines and is intended to deter future glitter-bombs on notable figures. Perhaps the court will play along and put the charge up for a hearing since it is a high profile case, but a full trial and conviction would set a super-dangerous precedent and therefore is very unlikely (an in my opinion imprudent and frivolous.)
There's my legal opinion for y'all.