A Question for the Ages

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I made a 2-color hat, with 1 ball of each color.  This is what was left over.  Hardly enough to donate to a bird's nest.  So here's the question:  Should I write the pattern to specify 2 balls of each color, just to keep my knitters safe from the horror of running short?  Can I, in good conscience, cause my trusting knitters to buy fully twice as much yarn as is needed?  What if the balls I used were unusually long though?  What if the next 2 balls from that mill would have been six inches short?  Weigh in, Gentle Readers:  Should my pattern specify 2 balls or 4? 

We who are about to knit salute you.