Colour cannot stand alone; it cannot dispense with boundaries of
some kind. [Footnote: Cf. A. Wallace Rimington. Colour music (OP.
CIT.) where experiments are recounted with a colour organ, which
gives symphonies of rapidly changing colour without boundaries--
except the unavoidable ones of the white curtain on which the
colours are reflected.--M.T.H.S.] A never-ending extent of red
can only be seen in the mind; when the word red is heard, the
colour is evoked without definite boundaries. If such are
necessary they have deliberately to be imagined. But such red, as
is seen by the mind and not by the eye, exercises at once a
definite and an indefinite impression on the soul, and produces
spiritual harmony. I say "indefinite," because in itself it has
no suggestion of warmth or cold, such attributes having to be
imagined for it afterwards, as modifications of the original
"redness." I say "definite," because the spiritual harmony exists
without any need for such subsequent attributes of warmth or
cold. An analogous case is the sound of a trumpet which one hears
when the word "trumpet" is pronounced. This sound is audible to
the soul, without the distinctive character of a trumpet heard in
the open air or in a room, played alone or with other
instruments, in the hands of a postilion, a huntsman, a soldier,
or a professional musician.

But when red is presented in a material form (as in painting) it
must possess (1) some definite shade of the many shades of red
that exist and (2) a limited surface, divided off from the other
colours, which are undoubtedly there. The first of these
conditions (the subjective) is affected by the second (the
objective), for the neighbouring colours affect the shade of red.