convert color name to color triple. Color name/triple combinations
were taken from the Unix X11/R4 rgb.txt file and converted to
a format matlab would properly interpret. This function was
created by Mickey P. Rowe some time around 1993 or 1994. In June of 2000
I found out that the switch statement could be used instead of successive
calls to strcmp. I suspect that the compiled version is identical, but this
looks more elegant.