Fix Ip / IMP color codes for plotting

[byersiiasa]

Please confirm that this PR has done the following:

• [ ] Tests Added
• [ ] Documentation Added
• [x] Name of contributors Added to AUTHORS.rst
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR fixed wrong color codes for AR6 Illustrative Pathways

[danielhuppmann]

Not sure why @jkikstra added the wrong colors in #650...?

Anyway, I'll start a PR into your branch updating the test-plots so that the tests pass.

[codecov[bot]]

Codecov Report

Merging #794 (dd2f463) into main (5b2f3b6) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #794   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
=====================================
  Files         62      62           
  Lines       5996    5996           
=====================================
  Hits        5681    5681           
  Misses       315     315           

Files	Coverage Δ
pyam/plotting.py	92.9% <ø> (ø)

[jkikstra]

Anyway, I'll start a PR into your branch updating the test-plots so that the tests pass.

@jkikstra also doesn't know! I assume I myself got the colours from @byersiiasa, so maybe I myself had an old version (e.g. SOD)?