Fix YELLOW_C and add PURE_CYAN, PURE_MAGENTA and PURE_YELLOW

[chopan050]

Fixes #3579

As suggested by @yuan-xy, this PR changes YELLOW and YELLOW_C from #FFFF00 to #F7D96F in order to make them more consistent with the other shades of yellow.

In order to not lose the original yellow, it has been renamed to PURE_YELLOW in the same fashion as PURE_RED, PURE_GREEN and PURE_BLUE. For extra consistency, this PR also adds PURE_CYAN and PURE_MAGENTA.

Here is the updated color table with the modified YELLOW_C and the extra three pure colors:

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested