Laminar Sciences Corp makes a very special rheoscopic fluid that shows fluid flow with a splash of rainbow colors (using birefringence).  We call it KaleidoFlow™ Rheoscopic Fluid.  Most of the pictures for this item show the fluid being used.

This is the "Professional" formulation of KaleidoFlow Rheoscopic Version.  This formulation exhibits better clarity and uses more refined materials than our "educational/hobbyist" formulation

Our KaleidoFlow Rheoscopic Fluid is not as simple to use as the standard mica-based rheoscopic fluids sold by others on the web (those other fluids are fine, but they don't do colors like ours does) and we don't want to sell it to anyone who's not up for some engineering challenge.  To produce the magic colors, our fluid must be backlit; be polarized on both sides; and come in thin tanks (0.25" to ~1.5" thick).  A design guide is available on our website.