Y’know, I always liked to think that the next day Dash would be arrested.  
Because fillies are full of blood and guts they would only produce one color: red. That would make all the rainbows red as well.  
I am sure that the princesses would notice this(along with everyone else in Equestria) and would immediately get to the bottom of it.  
Rainbow Dash would turn out to have been forced into that position, maybe by a mix-up or something, and ended up panicking due to not knowing anything about making rainbows. She would snap and end up killing fillies to make the rainbows, not knowing that it would be only red rainbows.  
She would only get to do it one time before everyone found out tho.  
But the princesses would then decide to supervise the process themselves to that this didn’t happen again. Since Celly only had the Pegasi do this because Luna was away, they would be able to do this without any problems.