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.

We have been selling this fluid to museums and tech-savvy "makers" for several years now.  We are now, admittedly, using the broad scale of eBay to try and reach more potential customers.   BUT, our 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.

Bottles of 4, 8, 16, and 32 ounces are available.  And we offer the 4 oz bottle with 2 - 4"x4" sheets of polarizing film as a kit.  Also a 8 oz bottle with 2 - 5"x5" sheets of polarizing film as a kit.   (but as of this instant, I am only posting the 4 oz bottle)