citrination

from The Century Dictionary.

• noun The process of becoming citrine in color; the state of being so colored. Also citronation.

from the GNU version of the Collaborative International Dictionary of English.

• noun The process by which anything becomes of the color of a lemon; esp., in alchemy, the state of perfection in the philosopher's stone indicated by its assuming a deep yellow color.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun archaic The process of taking on a lemon-yellow colour, as the philosopher's stone was supposed to do in alchemy.