In an article in Aerotechnews.com, Captain Ken Spiro, the chief intel officer for the 64th AGRS described how they come up with these threat paint jobs. Of interesting note is that the new "shark" scheme seen today has been in the works since at least August of 2016:

“The paint scheme is a means of representing threats more accurately... There are real-world threats that paint their jets in this way so we are changing over to make it more physically like their aircraft. Once a pilot who is training comes within visual range of the new Aggressor, they’ll be seeing a similar situation to what they would see with an actual threat aircraft... The idea started at the 64th AGRS because we’re always looking for different ways to be more threat representative, and make the training more realistic... The 64th AGRS gets creative in extra ways, such as paint schemes to accurately and better represent threats. We act like, look like, or anything you can think of we try so we can be true to the threats... We are working on a second ‘shark’ scheme because the 64th AGRS would like to see two threats eventually. That scheme is in the design stages right now and the preliminary designs have been sent over to Viper for their changes and approval.”

The idea of US aggressors sporting the shark scheme adorned with red stars is maybe a little more controversial than other paint jobs that have come and gone in the past as US-Russian relations have hit an all-time low, and the paint job is being flown on Russian jets in Syria today. But really, the fact that the 64th can jettison the political correctness to bring a richer threat representation to front-line fighter pilots underscores how big of a deal their job is, and it is more important now than ever.

Fighter pilots have been known to get "buck fever" the first time they see an enemy aircraft in real life. This distraction can lead to mistakes being made and to slower reaction times right when brevity is needed most. The 64th gets pilots accustomed to this phenomenon in a synthetic manner, so that on the battlefield they will be able to overcome their natural tendencies.