There are some skeins of eyelash yarn that are just the right size, but some of them don't have enough. You're short by about 2 inches with those skeins.
For example, I've used Lion Brand Fun Fur, and ran out too soon. But, Lion Brand Fun Fur Stripes was just right.
So, most of the time, I use more than one color. Leftovers in my eyelash yarn stash are always fun to use. The inconsistency actually adds character and interest.