Document requirements for a language color change (github-linguist#4487)

* Document requirements for a language color change

* Implement feedback suggestions

.github/ISSUE_TEMPLATE.md

@@ -10,6 +10,8 @@ Please confirm you have...
 
 <!-- Please review these preliminary steps before logging your issue. You may find the information referenced may answer or explain the behaviour you are seeing. It'll help us to know you've reviewed this information. -->
 
+<!-- Please review the "Changing the color of a language" section in CONTRIBUTING.md before asking for a language color to be changed. -->
+
 ## Problem Description
 
 <!--- Provide a more detailed introduction to the issue itself, and why you consider it to be a bug -->

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,10 +1,11 @@
-<!--- Briefly describe what you're changing. -->
+<!--- Briefly describe your changes in the field above. -->
 
 ## Description
 <!--- If necessary, go into depth of what this pull request is doing. -->
 
 ## Checklist:
 <!--- Go over all the following points, and put an `x` in all the boxes that apply. -->
+<!--- Feel free to remove whole sections, not points within the sections, that do not apply -->
 <!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
 - [ ] **I am associating a language with a new file extension.**
   - [ ] The new extension is used in hundreds of repositories on GitHub.com
@@ -41,10 +42,16 @@
   <!-- Update the Lightshow URLs below to show the new and old grammars in action. -->
   - Old: https://github-lightshow.herokuapp.com/
   - New: https://github-lightshow.herokuapp.com/
-  
+
 - [ ] **I am updating a grammar submodule**
   <!-- That's not necessary, grammar submodules are updated automatically with each new release. -->
 
 - [ ] **I am adding new or changing current functionality**
   <!-- This includes modifying the vendor, documentation, and generated lists. -->
   - [ ] I have added or updated the tests for the new or changed functionality.
+
+- [ ] **I am changing the color associated with a language**
+  <!-- Please ensure you have gathered agreement from the wider language community _before_ opening this PR -->
+  - [ ] I have obtained agreement from the wider language community on this color change.
+    - [URL to public discussion]
+    - [Optional: URL to official branding guidelines for the language]

CONTRIBUTING.md

@@ -157,6 +157,18 @@ If problems are found, please report these problems to the grammar maintainer as
 
 Please then open a pull request for the updated grammar.
 
+## Changing the color associated with a language
+
+Many of the colors associated with the languages within Linguist have been in place for a very long time.
+The colors were often chosen based on the colors used by the language at the time and since then users will have become familiar with those colors as they appear on GitHub.com.
+If you would like to change the color of a language, we ask that you propose your suggested color change to the wider community for your language to gain consensus before submitting a pull request.
+Please do this in a community forum or repository used and known by the wider community of that language, not the Linguist repository.
+
+Once you've received consensus that the community is happy with your proposed color change, please feel free to open a PR making the change and link to the public discussion where this was agreed by the community.
+If there are official branding guidelines to support the colour choice, please link to those too.
+
+Please note that Linguist currently implements a [color proximity test][] to ensure colors are sufficiently different from one another so you may not be able to use the precise color you want - reds and blues are really popular.
+As such, we recommend you test the color change locally before making your plea to the wider language community.
 
 ## Testing
 
@@ -227,5 +239,6 @@ If you are the current maintainer of this gem:
 [languages]: /lib/linguist/languages.yml
 [licenses]: https://github.com/github/linguist/blob/257425141d4e2a5232786bf0b13c901ada075f93/vendor/licenses/config.yml#L2-L11
 [new-issue]: https://github.com/github/linguist/issues/new
+[color proximity test]: https://github.com/github/linguist/blob/master/test/test_color_proximity.rb
 [samples]: /samples
 [search-example]: https://github.com/search?utf8=%E2%9C%93&q=extension%3Aboot+NOT+nothack&type=Code&ref=searchresults