You can remove some of your batch oil for mixing your colors. If you premix them with other oils you will just add some extra superfat or cut your superset down a point or so. I am not as exacting as some here. But I usually use glycerin for pre mixing color since it does not go rancid
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.
The reason I don't use my batch oils is because I never quite know how much color I want to use, and premixing with oils I find that quite often then go rancid even if I have used OO. I have often wondered if the minerals in the colorants accelerate rancidity, so I stick to glycerin or add micas staight into my just emulsified batter. Never had specks as long as I add the mica in when I just reach emulsification