I think that the color combination is very complimentary as the strong gold frames compliment the bold black background as well the dark outfits of the actors. I also think that there is a split complimentary aspect to this poster because the main character is wearing a black suit, which rather stands out from the black background because the two characters on either side are wearing dark colors, which instead compliment the main character, therefore creating more emphasis on him.
The color scheme helps to create a very grand and old school vintage appearance. This color combination is something that would be categorized as the classy and rich in the 20th century (this is what the filmmakers wanted to portray). The poster of the film seems visually focused on Leonardo DiCaprio and Carey Mulligan in spite of the presence of other characters because the movie is majorly based on their love story. The color combination provides a very sophisticated genre look, classy, mysterious, visually striking, emotional drama and thrilling.