Have you ever wondered why so many folk in the fashion industry wear black with black with black? Monochromatic dressing ensures you look slim, everything goes with everything and it's hard to tell the cost!
Personally, I disagree that wearing all black makes you look slim: you look heavier, it's hard to see your shape, and you're silhouetted against almost every background. Black can be hard to wear near your face, even if you are a Winter (and it's one of your neutrals).
Bend the rule slightly by adding a bit of colour. The simplest way is to wear your dark, base colour in your trousers/skirt and jacket/cardigan with a flattering colour in your scarf/blouse ie 'inside' your silhouette.
The other option is to keep your inner silhouette monochromatic (top & bottom) with colour introduced via a jacket/cardigan. While this will work, it isn't as versatile because
- a) jackets are usually more expensive than tops
- b) coloured jackets are more memorable (so you can't wear them as often) and
- c) we take our jackets off (losing the uplift colour).