The following method is for my dolls' nipple makeup. Just for your reference.
I used paint thinner to brush over the painted area to open up the surface for painting. It will keep the painting bond to the surface better. Mix a small patch of ecoflex 1A and 1B then add a little bit toluene to dilute it. Then add color pigment. Stir them well then apply on the nipple. A thin layer at a time. You need to apply maybe couple layers because the silicone has been diluted. After that use a dry brush dab around the nipple to create the soft edge effect. Let it dry and it's done. I use ecoflex as the body plus color pigment to paint. Silicone will stick with silicone. That's why I use ecoflex. If the silicone is too thick , after it dry it will looks shinny, you can use a Q-tip to dab on the nipple when it's still sticky, close to cure condition and it will not become shinny. Hope I can explain it well.