I was short of the grey (less than a full skein left), so I modified the pattern as follows:

-16 rounds of k1p1 rib to start (not 19)
-don’t start the thumb round until the end of the chart
-at the end of the chart, after the last main colour chevron, just carry on with the second colour. I knit 7 rounds of stst, then 4 of k1p1 rib, then cast off.