Acrylic paints are known for their fast drying time, wide range of colors, and relatively low cost. But unlike oil paints or watercolor, acrylics are water-soluble. They will thin out when mixing colors with water and become less opaque. This is why starting with a thick, heavy body of paint is essential. The wider the paint, the more opaque it will be. See here details https://loonycraft.com/how-to-make-acrylic-paint-more-opaque/