To do an initial fill cycle with your 7900 printer, you can either use the Adjustment Program with a PC computer, or preform an Initial Fill thru the serviceman menu on the printer's LCD panel, as described in the post above, and here: http://www.inkjetmall.com/tech/content.php?151-Initial-Fill-Procedures-for-Epson-Pro-Model-Printers
If you didn't flush color ink from your printer before installing Piezography inks, you will likely experience some color staining, and as you have noticed yellow is usually the most stubborn. If you have pure Piezography ink (no color staining) printing from all channels except yellow, then I recommend printing pure yellow purge sheets thru QTR Calibration Mode, by following our instructions, until the yellow color is purged and Piezography output is pure gray as it should be.
Pleas let me know if you have questions, or there's anything else I can help you with.
Best regards~ Dana