How use colors (#19)

* add first rst

* Add color tutorial

Author: Esteban82

docs/index.rst

@@ -113,3 +113,10 @@ Basics
       :align: center
 
    :doc:`tutorials/basics/region`
+
+-  .. image:: _images/42eb572e679e97489f2a5029a5d74352.png
+      :target: tutorials/basics/color.html
+      :width: 80%
+      :align: center
+
+   :doc:`tutorials/basics/color`

docs/tutorials/basics/color.rst

@@ -0,0 +1,96 @@
+How to use colors
+-----------------
+
+In GMT you will want to define the color of different features (line, symbols, polygones, text, etc.).
+There are six methods to define a color:
+
+* `Color names`_
+* `R/G/B`_
+* `#RRGGBB`_
+* `Graylevel`_
+* `H-S-V`_
+* `C/M/Y/K`_
+
+Color names
+~~~~~~~~~~~
+
+This is more friendly way to define a color. There are 663 unique color names that can be selected. 
+All names are case-insensitive. You can see the names (and their RGB values) in this 
+`link <https://docs.generic-mapping-tools.org/latest/_images/GMT_RGBchart.png>`_
+or use this interactive :gmt-module:`color-picker`.
+
+In the following example, we use ``lightgray`` for the dry areas and ``royalblue4`` for the wet areas.
+
+   .. gmtplot::
+
+      gmt begin color png
+	      gmt coast -Rd -JW12c -Glightgray -Sroyalblue4
+      gmt end show
+
+Color models
+~~~~~~~~~~~~
+Alternatively, you can also use color models to define a color. 
+These are useful especially when you know the values that define the color you want to use.
+GMT identifies the color model according to the syntax used.
+
+R/G/B
+=====
+
+Provide 3 values each from 0 to 255 separated by a /.
+Specify **R**\ed, **G**\reen, and **B**\lue levels. 
+Each value is separated by a slash and is in the range from 0 (dark) to 255 (light).
+This representation is used to color monitors.
+
+
+#RRGGBB
+=======
+
+Specify Red, Green, and Blue levels in the way that it is done in HTML. 
+Use two characters for each color channel, ranging from 00 (dark) to FF (light). 
+Upper and lower case are allowed.
+
+
+Graylevel
+=========
+
+Specify a single number from 0 (black) to 255 (white). It only uses shades of gray (R = G = B).
+This representation is popular with black and white printers.
+
+
+H-S-V
+=====
+
+Specify **H**\ue in the range 0 to 360 (degrees), **S**\aturation between 0 (not saturated) and 1 (fully saturated), and **V**\alue between 0 (dark) and 1 (light). Number are separated by hyphens. This representation can be helpful when hue varies a lot.
+
+
+C/M/Y/K
+=======
+
+Specify **C**\yan, **M**\agenta, **Y**\ellow, and blac**K**. Each number is in the range from 0 (no paint) to 100 (maximum paint). This representation is used by most color printers.
+
+
+Example with color model
+========================
+It is possible to make the previous map with color models.
+
+Color models for ``royalblue4``:
+
+* R/G/B: 39/64/139
+* #RRGGBB: #27408B
+* H-S-V: 225-0.719-0.545
+* C/M/Y/K: 72/54/0/45
+
+Color models for ``lightgray``:
+
+* R/G/B: 211/211/211
+* #RRGGBB: #D3D3D3
+* Graylevel: 211
+* H-S-V: 0-0-0.827
+* C/M/Y/K: 0/0/0/17
+
+
+   .. gmtplot::
+
+      gmt begin color png
+	      gmt coast -Rd -JW12c -G211 -S39/64/139
+      gmt end show