Add the link for standard GMT color names in GMT docs
Color land and water
--------------------
-The ``land`` and ``water`` parameters of :meth:`pygmt.Figure.coast` specify a color to
-fill in the land and water masses, respectively. You can use standard GMT color names or
-give a hex value (like ``#333333``).
+The ``land`` and ``water`` parameters of :meth:`pygmt.Figure.coast` specify
+a color to fill in the land and water masses, respectively. There are many
+:gmt-docs:`color codes in GMT <gmtcolors.html>`, including standard GMT color
+names (like ``skyblue``), R/G/B levels (like ``0/0/255``), a hex value (like
+``#333333``), and a graylevel (like ``50``).
"""
import pygmt
