If you're making your own design [hooray for you! What a neat adventure!] then you do have options.
You can always change the accent colors, adding more or less color as your yarn supply allows. Since this is Fair Isle, the pattens are reasonably small... most of mine run a max of 10-12 rows, with one or two 25-rowers.
Before starting one of the patterns/designs, I usually check that there is about one skein of the main color. Then the next pattern I check how much of that one's main color I'll need - again doing about a skein.
Here's a tip: more for your peace-of-mind, take your best guess as to how much yarn you'll need... be as precise as possible! ... and triple it.
The beauty of Fair Isle, for me, is that this is a perfect way to use up those odds-and-ends that I have lying around.
Here's another tip: for the pattern color, if you use a slightly heavier yarn than your main project's made of, then the scrunching-bunching is less. And your color stands out a tad better, too.
Hope this helps,
PS - if you need to get a feel for how much, try knitting a bag and practice using your Fair Isle patterns on that. And you'll have a bag that matches your outfit!