Cake sparkles are intended to give just that; a bit of twinkling color.  If used alone, the impact is subtle.  If you want to enhance the effect, use them in conjunction with a purple or lavender color underneath.  I use them with royal icing (it hardens) on cookies.  After spreading the icing, immediately sprinkle the sparkles on top.  The icing will hold them in place.  The two layers create a beautiful, richer look.  Think of a precious stone.  Without being cut properly, the stone won't sparkle.  But, with facets and light, it comes to life.  That's what the cake sparkles do for your flat icing.  I keep a supply of several colors and have never been disappointed with my finished products.