When you say a pattern like this, do you meen the pants themselves or the aptterns of the stripes in the fabric?
If it is the colour variation you are after, unfortunately that is going to be down to the yarn used and the way it stripes. It looks like somthing with fairly short repeats of colour is used to creat the effect. That is why it breaks up above the crotch/below the wasit, where the colour repeats are covering a larger span.
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