Yellowcake is "the product of the uranium extraction (milling) process; early production methods resulted in a bright yellow compound, hence the name yellowcake.
The material is a mixture of uranium oxides that can vary in proportion and in color from yellow to orange to dark green (blackish) depending at which temperature the material was dried (level of hydration and impurities). Higher drying temperatures produce a darker, less soluble material.
Yellowcake is commonly referred to as U3O8 and is assayed as pounds U3O8 equivalent. This fine powder is packaged in drums and sent to a conversion plant that produces uranium hexafluoride (UF6) as the next step in the manufacture of nuclear fuel." 
- http://www.nrc.gov/reading-rm/basic-ref/glossary/yellowcake.html, Nuclear Regulatory Commission, "Yellowcake" (2007).