Lead carbonate is a clear, toxic chemical that was put in paints before 1930 to make them white. The speed of light in a particle of lead carbonate isn't that much slower than in plastic. Since light changes speeds only slightly in entering or exiting from the lead carbonate particles, the partial reflections from the particle surfaces are weak. It takes many, many particles to send most of the light backward so that it doesn't pass through the paint layer. But with a weak reflector like lead carbonate, the light must pass through many, many particles before most of it has been reflected, so the paint layer must be very thick.
Modern paints contain particles of clear, nontoxic titanium dioxide.
Light travels very slowly in this material. Explain why titanium dioxide based paints look so white, even after only a single layer of paint. Since light changes speeds dramatically in entering or exiting from titanium dioxide, the partial reflections from the particle surfaces are strong. Only a few reflections are needed to stop most of the light so even a thin layer of paint is needed. Each surface reflection is strong, so not many are needed to stop the wave almost completely.