I have mixed barium sulfate with white paint, for making cheap reflective integrating boxes. The tricky part is maximizing the amount of powder to paint, without it getting too thick for painting. I have also used the Labsphere, integrating sphere white paint, it is also so expensive: https://www.labsphere.com/labsphere-products-solutions/mater...

On another note, the building rooftop outside my office window was painted white, within 1.5 years its gone from from blinding white to a very dark gray, I am not sure this does much good over time.