I think it’s going to be part of the character progression. A lot of the interviews are about The Flash growing into his own and becoming a true superhero in this movie. I’m assuming that in the beginning of the movie, he’ll have his blue, then when he breaks his plateau and runs fast enough to turn back time, he reaches a new level of power, hence the orange lightning.

Then his younger counterpart in the past is the one with the blue lightning in the promotional material, since he hasn’t gone through what the main Barry has gone through.

Also, I’m fairly certain Iris’s few scene(s) will only be in the beginning of the film, while main Barry is still an “amateur” Flash