I want to put paint inside a bottle but only do partial coverage...

Maybe use an eye dropper to help place the paint.

Just drip a teaspoon of paint at a time into a clean, dry bottle held at an angle. Twirl, tilt or roll the bottle until the paint is like you want it. Repeat with each teaspoon until you are pleased and decide it's finished. Let it dry. Best wishes 😇

You can use a turkey baster and put the paint in the bottom then swirl to cover the area that you want

Thank you all for the suggestions. Since I am using small bottles, I think the eye dropper will work the best. Sorry I didn't specify the size bottle I was using.

Only use enough paint to barely cover since you won't be able to pour it out without ruining it.