It isn't (you can find it in the Tk documentation, not perlTk). It's one of the parts of Tk i abhore. You can use the apparently case sensitive search at http://perltk.org to find more info, but what you want to pass is either basic names like red/green/blue, or hex strings ( #ff0000 #00ff00 #0000ff ).

http://perltk.org/ptknews/search.cgi?Search=Tk_GetColor yielded http://perltk.org/ptknews/21338.htm

From Tk_GetColor docs: 

colorname 
Any of the valid textual names for a color defined in the server's color 
database file, such as red or PeachPuff. 

#RGB 
#RRGGBB 
#RRRGGGBBB 
#RRRRGGGGBBBB 
A numeric specification of the red, green, and blue intensities to use to 
display the color. Each R, G, or B represents a single HEXADECIMAL digit. 
The four forms permit colors to be specified with 4-bit, 8-bit, 12-bit or 
16-bit values. When fewer than 16 bits are provided for each color, they 
represent the most significant bits of the color. For example, #3a7 is the 
same as #3000a0007000. 

MJD says you can't just make shit up and expect the computer to know what you mean, retardo! ** The Third rule of perl club is a statement of fact: pod is sexy.