[spoiler=Robin's alternate costumes]
[spoiler=Lucina's alternate costumes]
Yup, screenshots of Robin's and Lucina's alternate costumes, the latter of which are derived from other characters from Fire Emblem Awakening! Share your thoughts here! For the case of Lucina, from left to right, there's the default costume, Nowi, Cherche, Cordelia, Tiki, Lissa, Tharja and Sumia!
EDIT: added in Robin's alts. and hid the screenshots because of spoilers.
My thoughts on their costumes: by default, I would use the default costumes. However, in case the mood strikes me, or if it becomes apparent that visibility is becoming an issue and I don't want to put up a thick outline for the characters, I like green, red, blue and yellow costumes on Robin. For the case of Lucina, I actually like all of them since I can see the amount of effort Sakurai and his team put into the costumes (personal bias towards Cordelia, Tiki, Cherche, Lissa and Sumia).
By the way, doesn't Lucina in the Nowi alt costume look a lot like Nino from FE7?
... And can't believe that I just saw this now... male Robin in his white costume makes him look a lot like Kirito in his Knights of the Blood Oath uniform from Sword Art Online.