The font and color directives can be placed anywhere in the Chord file, though it is usefull to place them at the beginning :-). The specified values are valid for the whole document!
LaTeX allows the three font sizes 10pt, 11pt and 12pt. GuitarTeX sets the default value to 11pt (titles and subtitles are enlarged automatically). If you want a font size of 12pt you should write:
Note: do not write 12pt instead of 12!
The color directives allow to change the text color of the chorus, bridge, instrumental parts, tablatures and a second voice. For the second voice even the background color may be specified. So you can write white letters on a colored background for example. This will help the singers not getting confused by many lyrics lines :-)
The directives are named
color_chorus
color_bridge
color_instr
color_tab
color_second
color_second_back
The parameters must be a comma separated list of three numbers between 0 and 1, giving the red, green and blue components of the color.