Certain acrylic colors are more opaque than others - for instance, the cadmiums (yellow, orange, red, etc) tend to be more opaque than the quinacridones (red, magenta, etc).

One coat of white acrylic is generally not opaque enough to thoroughly cover any color underneath it, so you'll need to paint sometimes two or three coats to get the strong white opacity you may be seeking. This depends on how you apply it though of course - if you use a thick impasto technique, then the white will more easily cover anything underneath it. If the white acrylic is more diluted with water, then it will be even more translucent and you'll need to use more layers.

To increase the opacity of your white acrylic paint, try using it straight from the tube, with minimal (or no) water added.

Also, keep in mind that "Student Grade" acrylics will have more filler and less pigment, while "Artist Grade" or "Professional Grade" acrylics will contain more pigment and less filler. This means that Artist Grade acrylics will be more opaque than Student Grade acrylic.