While form is absolute,
so that you can say at the moment you draw any line that it is either right or wrong,
colour is wholly relative.
Every hue throughout your work is altered by every touch that you add in other places;
so that what was warm a minute ago, becomes cold when you have put a hotter colour in another place,
and what was in harmony when you left it,
becomes discordant as you set other colours beside it;
so that every touch must be laid, not with a view to its effect at the time,
but with a view to its effect in futurity,
the result upon it of all that is afterwards to be done being previously considered.
You may easily understand that, this being so, nothing but the devotion of life,
and great genius besides, can make a colourist.