This pull request contributes to issue #6207, which is to add more unit tests to the project.

Tests added:

ColorUtilTest
DialogWindowStateTest
RankingFieldComparatorTest
SpecialFieldComparatorTest

• Change in CHANGELOG.md described in a way that is understandable for the average user (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.