We cannot divide these collors because, being vibrational, they run
under the law of quanta and relativity rules. Dealing with colored
light is harder to distinguish limits betwen them, , first because it
is reflected light, second everyone has a diferent deep of perception
for the perception of colors.( adictive synthesis) If you talk of
pigmented colors, I mean colored inks, it is easier to stabilish a
diference.( subtractive synthesis) Even so weŽll find the same
differences in colorimetric perception among people. In rainbow, for
example, there are areas, bands, of possible colors but not specific
colored lights or radiations. I gess there is a confusion lying the
right meaning of "Arbitrary" .
OK. Putting in "indigo" into the colors is an arbitrary way of making 7
out of six more general colors. But showing the point where one color
ends and the other begins is virtually impossible. It is an attempt to
put in divisions on a continuity; any attempt to do so is automatically
arbitrary.