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 other projects like this in my book, The Chic Girl's Guide to a One-of-a-Kind Wardrobe!