You can approach this with rules rather than instinct. Why some combinations never fail.
The most common reason an outfit looks busy is simply too many colours. Holding the whole look to three drops the difficulty sharply. Counting shoes and bags as colours matters: two garments plus a bag and shoes in different colours is already four.
Black, grey, navy, beige and white work with nearly everything and clash with nothing. Filling most of the outfit with these and adding a single colour almost never fails. When sorting a wardrobe, more neutrals means far more workable combinations.
Two strong colours over equal areas fight each other. Using one broadly and the other in a small area makes the same pairing look deliberate. A bright colour in a small area draws the eye, so place it where you want attention.
If choosing colours is hard, varying only the lightness of a single colour is the safest route. Pale blue with deep blue removes the choice entirely. Mixing materials with different textures keeps it from looking flat.
Clothes bought online arrive looking different because screens reproduce colour differently and shoot under different lighting. Deep navy against black, and beige against white, are especially hard to tell apart in photographs. When colour matters, look at several images and favour photos where the item appears beside other garments.
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