Using the core of a toroidal inductor as the core for a toroidal transformer?
Powdered iron cores designed for EMI mitigation are lossy and may not work that well. If there are markings on the core you may be able to look up the material characteristics (and, of course, you can measure the core dimensions). If there is a standard color code for cores I'm not aware of it.  

You can test the Al by winding some turns on the core and measure the resulting inductance. Divide the uH you read by the square of the number of turns you added to get the Al value.