It doesn't look like much in the photo, but this past year I've come to really love it. W&N iridescent white acrylic, that is. (Buy Direct)
The way light falls onto a painting influences what we see; even more so when you've mixed a little iridescent white into your acrylics so that at certain angles the paint glimmers. Think sunlight on a spoon, reflections on water, sparkle on a glass, or simply shimmering colors in a forest floor. If you're working with oils, you can use the effect by underpainting with acrylics, letting it show through.
You can take it even further with Golden's Interference Colors (Buy Direct) where the color you see depends on the angle you're viewing the painting at, "flipping" a bright opalescent color and its complement.