g each pair:--

Red and yellow unite to form yellow-red (YR), popularly called
orange.[17]
Yellow and green unite to form green-yellow (GY), popularly called
grass green.
Green and blue unite to form blue-green (BG), popularly called
peacock blue.
Blue and purple unite to form purple-blue (PB), popularly called
violet.
Purple and red unite to form red-purple (RP), popularly called plum.

Using the left hand again to hold colors, the principal hues remain
unchanged on the knuckles, but in the hollows between them are placed
intermediate hues, so that the circle now reads: red, yellow-red,
yellow, green-yellow, green, blue-green, blue, purple-blue, purple, and
red-purple, back to the red with which we started. This circuit is
easily _memorized_, so that the child may begin with any color point,
and repeat the series clock wise (that is, from left to right) or in
reverse order.

[Footnote 17: Orange is a variable union of yellow and red. See
Appendix.]

(59) Each principal hue has thus made two close neighbors by mixing with
the nearest principal hue on either hand. The neighbors of red are a
yellow-red on one side and a purple-red on the other. The neighbors of
green are a green-yellow on one hand and a blue-green on the other. It
is evident that a still closer neighbor could be made by again mixing
each consecutive pair in this circle of ten hues; and, if the process
were continued long enough, the color steps would become so fine that
the eye could see only a circuit of hues melting imperceptibly one into
another.

(60) But it is better for the child to gain a fixed idea of red, yellow,
green, blue, and purple, with their intermediates, before attempting to
mix pigments, and these ten steps are sufficient for primary education.

(61) Next comes the question of opposites in this circle. A line drawn
from red, through the centre, finds its opposite, blue-green.[18] If
these colors are mixed, they unite to form gray. Indeed, the centre of
the circle stands for a middle gray, not only because it is the centre
of the neutral axis between black and white, but also because any pair
of opposites will unite to form gray.

[Footnote 18: Green is often wrongly assigned as the opposite of
red. See Appendix, on False Color Balance.]

(62) This is a table of five mixtures which make neutral gray:

{ Red & Blue-green }
{ Yellow Purple-blue }
Opp