And that lesson is:
You will run out of yarn.
If you think you just might have enough yellow to make a pair of longies if they're striped, but it will be close... you don't. You will run out of yarn. You will run out when you only have a little more to go. You will have just enough left to do maybe half of that last bit.
So you will save that last bit, and take it to the store, only to find out that they're out of that color. It's on order. It should be here by the end of the week.
So if you think you just might have enough left, you don't. Make the longies with 3 colors, just in case.