There is no evidence that Tyrosine reverses gray hair.
More Info: Gray hair is caused by a natural build up of hydrogen peroxide in the hair shaft, a natural occurrence as you age. When the hydrogen peroxide accumulates it damages the melanin which is responsible for giving your hair its color.
Catalase Production Inhibited
Every cell contains hydrogen peroxide that in younger individuals is immediately broken down into water and oxygen by the enzyme catalase. As you age, the body’s production of catalase slows down allowing the hydrogen peroxide to build up and damage the melanin in the hair shaft. 
What Does Tyrosine Have to Do with Gray Hair?
Tyrosine is a non-essential amino acid that among other important functions helps the body produce melanin. The higher levels of hydrogen peroxide that occur as a result of lower levels of catalase result in lower levels of tyrosine as well inhibiting its ability to produce melanin. 
Commercial Pill to Reverse Gray Hair
The commercial pills available claiming to reverse gray hair are mainly catalse supplements with additional tyrosine. The manufacturers market on the premise that if these enzymes are responsible for gray hair, then it follows that increasing production will reverse gray hair.
To date, there are currently no medically recognized studies to prove or disprove this theory.
 “The Vigilante Hypochondriac: The Reason for Gray Hair May Offer Insight Into Cancer Prevention.” University of South Florida Department of Dermatology. University of South Florida, 28 Oct. 2009. Web. 20 June 2011. <hscweb3.hsc.usf.edu/health/medicine/dermatology/?p=109>.
 “Tyrosine.” University of Maryland Medical Center | Home. N.p., n.d. Web. 20 June 2011. <http://www.umm.edu/altmed/articles/tyrosine-000329.htm>.