Some artists use a physics phenomenon called blueshifting to indicate which versions of the Flash are faster than other speedsters.

Ever since the return of Barry Allen in Final Crisis #2, DC Comics fans have speculated on which Flash truly deserves the epithet of “Fastest Man Alive.” On multiple occasions, it’s been confirmed that Barry Allen is in fact the origin of the Speed Force, a cosmic force which both he and Wally draw their power from. Due to this, one might think it would be logical that Barry - as the progenitor of such a force - would in fact be capable of reaching the fastest speeds imaginable. However, work by artist Brett Booth provides a visual cue which proves which Flash is fastest.

It has been confirmed multiple times that Wally West is the fastest Flash. Most notably, in the comic The Flash (2016) #49 Wally himself outruns both Superman and Barry Allen. In the following chapter, #50, Barry even admits that Wally has always been more in touch with the Speed Force than him, and officially passes the title of “Fastest Man Alive” to his once protege. However, Barry’s mention of Wally being more in touch with the Speed Force is important to note because it confirms an awesome detail about Flash’s Speed Force Lightning.

It’s no secret that once every Flash takes off, their body seems to become surrounded in a crackling aura of lighting. Depending on which Flash is running, however, the lightning surrounding them will change color. In answer to fan questions on Twitter, former Flash artist Brett Booth discussed how the concept of "blueshifting" applies to the Flash's powers in his and some others' depictions. Blueshifting refers to a change in the frequency of a photon wave depending whether or not it is moving towards or away from the viewer, with a blue object moving farther away and a red one closer. Booth took this concept and applied it to each Flash by changing the color of their Speed Force lightning based on how fast they are, stating, "so the blue means [Wally is] tapped into the Speedforce and is faster than present Barry."

By this reasoning - and as applied in Flash #50, from Howard Porter and Joshua Williamson - Wally West has blueish white lightning to signify that he is moving away from the viewer faster than any other Flash. Barry Allen has yellow lightning meant to signify that while he is fast, he’s nowhere near as in touch with the Speed Force as Wally. Additionally, Wallace West has red lightning, signifying that he remains currently the slowest of the bunch (but presumably has plenty of room to grow.) This awesome detail also draws fan attention to the fact that, in his New 52 incarnation, Barry Allen’s lighting is initially red and ends up blue in the future, showing that over time Allen becomes much faster.

Brett Booth’s explanation leaves fans with a plethora of questions, such as why Professor Zoom generates black lightning or what Wally’s manifestation of white lightning meant. Though Booth touched on the white lightning possibly meaning Wally was speeding up time itself, he left Zoom’s black lightning a complete mystery. Booth's explanation demonstrably illustrates the difference in speed between each Flash and could even be used to denote their speed hierarchy long into the future of DC Comics, potentially allowing readers to discern speed through this awesome visual shorthand.

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Source: Brett Booth