Funny, I was thinking of this thread earlier tonight.
I needed to do some quick touch ups on a playfield. I grabbed my acrylic paints and brushes, then looked for a Dixie cup or whatever to put a drop in from the container. Always a nuisance to find something small and clean to put acrylics in. I never seem to have a suitable container. Just then I look on my workbench and poof, the solution revealed itself: the leftover plastic insert to a box of 555 bulbs. Perfect. 10 little, clean troughs I can put paint in and mix up samples. Simple and stupid, but oh so effective. And there's always one laying around.