ppc_plot specify colors for each element

[aloctavodia]

Now we can pass a list of 3 colors instead of a single string

[codecov]

Codecov Report

Merging #1769 (0cc3483) into main (189b4da) will increase coverage by 0.04%.
The diff coverage is 94.02%.

❗ Current head 0cc3483 differs from pull request most recent head febef45. Consider uploading reports for the commit febef45 to get more accurate results

@@            Coverage Diff             @@
##             main    #1769      +/-   ##
==========================================
+ Coverage   91.17%   91.22%   +0.04%     
==========================================
  Files         115      117       +2     
  Lines       12254    12449     +195     
==========================================
+ Hits        11172    11356     +184     
- Misses       1082     1093      +11     

Impacted Files	Coverage Δ
arviz/plots/ppcplot.py	91.54% <66.66%> (-2.30%)	⬇️
arviz/plots/tsplot.py	91.66% <91.66%> (ø)
arviz/data/inference_data.py	83.76% <100.00%> (+0.04%)	⬆️
arviz/plots/__init__.py	100.00% <100.00%> (ø)
arviz/plots/backends/bokeh/ppcplot.py	98.11% <100.00%> (ø)
arviz/plots/backends/matplotlib/ppcplot.py	98.07% <100.00%> (ø)
arviz/plots/backends/matplotlib/tsplot.py	100.00% <100.00%> (ø)
arviz/utils.py	89.21% <100.00%> (+0.07%)	⬆️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 189b4da...febef45. Read the comment docs.

[ahartikainen]

Everything looks ok, but where is the test for making sure that colors list has atleast 3 items?