You can use different textures of the same colour to make your outfit more interesting. Even if the colour of your trouser and top is the same, but the variation in textures would give the outfit a rich interesting dimension and relief from sameness. With textures like ribbing, knits, tweeds, and jacquards type fabrics; you can even add patterns without ever getting near another colour.

If you do not feel like dressing in one colour then you can use a little variation in the colour palette however don't lose the monochromatic magic. For example mix subtle dark colors, such as navy and deep loden green, mink brown and charcoal gray, deep wine and raisin, or any of the above colour with black. The slight variations in color add interest, but you get almost the same solidarity as a single color since the eye is not terribly distracted and still produces a strong unbroken vertical line.

You can also blend lighter and darker shades of one color to give that monochromatic look to your outfit. The most slenderizing combination is to wear the darker tones on your lower half with the lighter tones nearer your face. Blends in the same color family give the impression of solid colour unity and make for a longer and leaner look. Plus, they are quite chic and very sophisticated to boot.