I know I read somewhere that a few drops
of black paint in a can of white will help to keep the paint from yellowing over time.
Is this correct??
Yes, a few drops of lamp black per gallon can help keep white paint white. The yellowing problem mostly affects oil based paints and not good acrylics. Acrylic paints are more color stable and a good choice for the color white.
Your local paint store can add the correct amount of black per gallon and still maintain a white color.