Love that hat pattern...it was my first hat!
If the colors of yarn you have for the hat look good together, go for the two colors in your hat. You can just start with the new color if there's enough contrast to look like you meant it to be half one color and half the other.
Or you can frog and knit a couple of rows in one color and a couple in the other, if they'd "blend" in that way.
But if you're not going to love the hat, I'd go for getting enough yardage in the color you want.
If you're going to return the yarn, of course, don't use it!