Read the colour from a distance
Hang up the shirt and step back. Fine blue and white stripes may read as a pale blue area from farther away, while broad dark stripes remain clearly separate. Choose the lower garment according to the overall colour you actually see. The precise name of the blue matters less than the direct comparison.
Navy trousers can repeat one colour from the pattern. Brown or cream trousers create another connection. Hold the pieces together in daylight and compare one lower garment at a time. Changing several things simultaneously makes the decision unnecessarily difficult.
Opening or closing changes the patterned area
Buttoned, the shirt presents one continuous area. Open over a plain T-shirt, it divides into two narrower sections. If the pattern initially feels too prominent, try the open version. The T-shirt beneath should remain visible enough for that interruption to look deliberate.
A tucked hem shortens the visible striped area. A loose hem lengthens it. Check both with the same trousers and look from the side. The aim is not to visually correct the body, but to arrange fabric areas according to your taste.
Add other details quietly or deliberately
Simple shoes and a quiet bag leave the shirt as the focus. If you want a second pattern, begin with a small visible piece such as a scarf rather than another large area. Related colours can help connect them without being compulsory.
With a jacket, inspect the collar and cuffs. A narrow striped collar may already show enough pattern. You do not need to expose the entire shirt hem at the same time. Remove the jacket and put it on again so the arrangement works in everyday use.
Do not turn direction into a body rule
Stripes do not need to lengthen or narrow a body to be wearable. Their direction is first a property of the fabric. A horizontal detail on a pocket can look interesting beside vertical shirt stripes because the lines change deliberately. Decide whether you like that change rather than treating it as a mistake.
With a shirt using panels in different directions, plain trousers can reveal its construction more clearly. A jacket may hide part of that detail. This is fine if the remaining view works for you. Clothes can show different portions of their design in different situations.
