Something that's prismatic reflects or resembles the multiple colors of the rainbow. The prismatic hues on the surface of the water brightened the gloomy day.
You can use this adjective to describe something that looks like a geometrical prism, a three-dimensional shape with two parallel bases and sides which are all parallelograms. The tent you use when you go camping is prismatic in shape, for example. Prismatic also means "reflecting light the way a prism does," so you can describe the prismatic light that shines through ancient stained glass windows in a church.