Add the link for standard GMT color names in GMT docs (#1049)

Co-authored-by: Meghan Jones <[email protected]>
Co-authored-by: Dongdong Tian <[email protected]>

Author: 3 people

examples/gallery/maps/land_and_water.py

@@ -2,9 +2,11 @@
 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