For the longest time I would spray some paint in the spray can cap, dip my brush and touch up a mistake.
But after a few sprays in the same cap, new paint could dissolve some of the previously dried paint. The brush could pick up the old paint and it was transferred to the model.
I tried spraying directly into some small mixing cups, but the paint ate right through the plastic cup!
I lined the cup with a plastic snack bag. Be sure to press the bag all the way into the bottom of the cup and roll the edges over the top of the cup.
Spray right into the bag in the cup and paint.
When finished, pull out the bag, seal closed and throw away. You can reuse the cup.
I recently bought cheap brushes with a 40% off coupon at Michaels crafts.
These are not great brushes, but are good for touch ups.
Even with a good thinner, then water and soap, some residual paint remains. If you were to use that brush for a white touch up after using the same brush with black before, your touch up paint could come out gray!
These real hair brushes were cheap enough, I now have a single brush for different colors. I mark the rear of the handles so I will only use them for one color.