I have my main colors premixed with olive oil in squirt bottles, including AC and TD. Less messy, less stuff to wash (specially for multiple colors), and saves me time. I mix it thick so that it's almost the consistency of ketchup. Even for darker color soap I might add only 2 grams of olive oil per kilogram of oils. That's like 0.2% of my oils and not worth worrying about.
If you take oil from your batch and mix, you really don't know if you've added enough or too much color until you mix it with the soap batter you're trying to color. At least for me, it would be harder to gauge the amount or shade of color I'm trying to achieve. I find it better for the way I work to squirt directly into my soap batter, stir with spatula, see the color, then add more if needed. If I need perfect consistency, I measure by gently squirting it into a teaspoon.