Merge identical group colors

[WangAooa]

Fixes #6175
Hi, I made some mistake that close the PR. This is a new PR about #6175 issue.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[calixtus]

Hi @WangAooa , thanks for reintroducing this PR. However, @tobiasdiez asked for some changes in the other PR. Please fix the checkstyle issues and consider the changes he asked for.

[calixtus]

#6491 (review)

[koppor]

This is a good start. Since there was no activity since a few weeks, we would like to ask whether there is some interest in continuing in this.

[calixtus]

Hi @WangAooa , since there was no progress here and we like the idea, we gonna take this over and finish it soon. Thanks for your work.