Replace `color` for `fill` in PyGMT plots

[santisoler]

Description of the problem/suggestion:

While building the docs, PyGMT is raisin a FutureWarning about the deprecation of the color argument in favor of the fill argument:

FutureWarning: The 'color' parameter has been deprecated since v0.
8.0 and will be removed in v0.12.0. Please use 'fill' instead.  

We should update the User Guide that generate plots using the color argument and use fill instead.

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