You can match the colors as I did for this project or make the peplum a different color entirely (think graphic color blocking). The main thing is to make sure the peplum length flares gently over the hips--it shouldn't be any longer than six to eight inches, depending on your height. Too long and it's a tunic--too short and it's a tutu (and we wouldn't want to look like a lost ballerina, after all).
Cropped top from H/M
I cut the rectangle of fabric on the bias rather than straight on the grain to create that slight fluttery effect.
See more in my book, The Chic Girl's Guide to a One-of-a-Kind Wardrobe