add color and alpha arguments to plot methods

[aloctavodia]

• Code style is correct (follows pylint and black guidelines)
• Includes new or updated tests to cover the new feature

[codecov-commenter]

Codecov Report

Merging #240 (e87376a) into main (eaa8bdf) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   82.82%   82.83%   +0.01%     
==========================================
  Files          29       29              
  Lines        3674     3677       +3     
==========================================
+ Hits         3043     3046       +3     
  Misses        631      631              

Impacted Files	Coverage Δ
preliz/tests/test_plots.py	100.00% <ø> (ø)
preliz/distributions/distributions.py	91.30% <100.00%> (ø)
preliz/internal/plot_helper.py	72.90% <100.00%> (+0.26%)	⬆️