The reason is because smoke trailers using missiles have tendency to blow up in maneuvers for example i was recently flying Hyperloop's SU-30 and I tried to put smoke trailers on the wings and than using the thrust vectoring tried to do a cool trick while the smoke was on but the missiles just blew up in the G force so thats why we actual smoke trailers PS the change color part is just for show and would be cool

Here is the Sukhoi
https://www.simpleplanes.com/a/blvS12/3D-Vectoring-Sukhoi-Su-30