As the Speed Force has become the new villain for The Flash season 7, the cosmic force's various lightning colors reveal more about the evil twist. Ever since Iris West revived the original Speed Force, the source for all speedsters has been a physical being for the last few episodes. As they have been using Nora Allen's form, they have been trying to help Team Flash stop the New Forces, while also having a hidden agenda of their own. In the eighth episode, "The People v. Killer Frost," the Arrowverse drama introduced the Strength Force's human conduit Alexa Rivera, a.k.a. Fuerza, who showed up in the fourth episode. While Barry Allen managed to convince Alexa to let them do tests on her, the Speed Force seized the opportunity to take out the Strength Force conduit.
But as the Speed Force took its role as a rival to The Flash, the plot twist may have actually been foreshadowed ever since they returned. Whenever the Speed Force would be seen using their powers or running next to Barry, they would emit more than one lightning color, which is standard for most speedsters. But since the Speed Force is the source for their speed, it makes sense that they project multiple colors too. However, one of the three lightning colors that indicated they were up to no good, was the red one. Red lightning is mainly associated with the Reverse-Flash's Negative Speed Force, but also with Nora West-Allen after she tapped into it in The Flash season 5.
The Speed Force having red lightning doesn't necessarily imply that they're connected to the Negative Speed Force. Ever since Eobard Thawne's consciousness was removed from Nash Wells' body in The Flash season 6, it's unclear if the Negative Speed Force still exists at this point. Given that there haven't been any good speedsters that emit red lightning, it's fascinating that red is one of the Speed Force's lightning colors, alongside blue and the traditional yellow - as this did preemptively suggest the Speed Force did have some hidden malicious intent.
Whenever a speedster is emitting blue lightning, it indicates their speed isn't organic. A speedster who has generated blue lightning was Hunter Zolomon, a.k.a. Zoom, and Eliza Harmon, a.k.a. Trajectory, in The Flash season 2. They didn't get their speed naturally and instead used Velocity drugs, enabling them to become speedsters. While it would connect them to the Speed Force, using Velocity came at a cost as it would rapidly cause cellular degeneration. As of The Flash season 7, the Speed Force becomes the first speedster to ever generate blue lightning without having enhanced themselves with Velocity.
With the Speed Force established as Team Flash's latest antagonist, their lightning generation becomes an even bigger mystery. Even though XS emitted both yellow and purple lightning, Barry and Iris' respective colors, seeing the cosmic entity generating three of them is fascinating. It could symbolize that when they were reborn, they came back differently, a problem that maybe Barry has to fix. They're obviously not going to kill the Speed Force as The Flash, and other speedsters, still need their powers. The likeliest conclusion to this storyline is The Flash and his team having to find a way to undo whatever change that corrupted the Speed Force to be acting this way.