I use tester as well but I thin it down quite a bit with thinner and then 'flow' it into the places that need it. I take a coffee stir straw and dip it into the thinned down paint and then place my finger on the back of the straw. I then pull the straw out and paint is held in the straw as long as my finger is on the back of the straw. I then place the tip of the straw where I want the paint and take my finger off the end. The paint then flows out of the straw. It flows to all the edges of the cavity. This technique avoids brush stroke and it looks quite even. I must note that the item you are coloring must be level and the cavity must be position up. Let the paint dry for a few hours, then you can go back with a tooth pick and or clean cotton cloth and rub out any places where paint got to where you did not want it to be. It will take much longer for this to fully dry since you will have a lot of thinner that need to evaporate. I give it about 2 days before I move it just to make sure it is hard and will not get scratched or messed up by something hitting the soft paint.
