The only always successful monochromatic look anyone can pull off is when they wear all black. Even then, there might be the issue of deciding if you are a character from The Matrix. It is harder to match colors of a different shade, and sometimes questionable if you are the epitome of the variation of colors from head to toe. Ever see someone wearing light blue to dark blue all over? How many times was it a success?
Rihanna visited the color wheel right before she made her appearance on 106 and Park for BET Studios (so everyone is visiting New York right about now!) promoting her new album, Loud. Her large knit sweater matched the pastel green of her leggings. The light color of the leggings verges on being unflattering on her (let alone people who don't train or perform regularly!) and could have used a pattern, embellishment, or even a thicker material.
She did adhere to the "tight on the bottom, loose on the top (and vice versa)" rule, keeping her from turning into an ice cream colored blob, but I feel like she could've done better. The look came off leisurely and relaxed (the straight hair also added to the look, because it wasn't as fancy as
we I expected), but was inches away from being a bit of a disaster.
To copy the look, try darker leggings or tight jeans/pants instead. Embellishments, like black trim along the sides will deflect from any problem areas you have in your bottom half (I know I have some dimples that no leggings will care to hide).
Here are some ideas: