Example:

// A solid white line segment
const primitive = new Cesium.Primitive({
  geometryInstances : new Cesium.GeometryInstance({
    geometry : new Cesium.SimplePolylineGeometry({
      positions : Cesium.Cartesian3.fromDegreesArray([
        0.0, 0.0,
        5.0, 0.0
      ])
    }),
    attributes : {
      color : Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 1.0, 1.0, 1.0))
    }
  }),
  appearance : new Cesium.PerInstanceColorAppearance({
    flat : true,
    translucent : false
  })
});

// Two rectangles in a primitive, each with a different color
const instance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(0.0, 20.0, 10.0, 30.0)
  }),
  attributes : {
    color : new Cesium.ColorGeometryInstanceAttribute(1.0, 0.0, 0.0, 0.5)
  }
});

const anotherInstance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(0.0, 40.0, 10.0, 50.0)
  }),
  attributes : {
    color : new Cesium.ColorGeometryInstanceAttribute(0.0, 0.0, 1.0, 0.5)
  }
});

const rectanglePrimitive = new Cesium.Primitive({
  geometryInstances : [instance, anotherInstance],
  appearance : new Cesium.PerInstanceColorAppearance()
});