These examples all end in اء:
- حَمْرَاءُ، صَفْرَاءُ، خَضْرَاءُ
- عَرْجَاءُ، بَكْمَاءُ
- صَحْرَاءُ
They also illustrate different uses: صَحْرَاءُ is a noun, رَغْبَاءُ is a verbal noun, and زَرْقَاءُ and عَرْجَاءُ are adjectives. The shared point is the feminine mamdudah ending, not a single word class.