You can add Function object to your project and create a function that resets font color for all texts:
On Function "SetAllTextBlack" -> FamilyTextObjects set Font Color to rgb(0,0,0)
Then on tap call this function before setting yellow color to tapped text:
Touch On Tap FamilyTextObjects
------------------------> Function call "SetAllTextBlack"
------------------------> FamilyTextObjects set Font Color to rgb(255,255,0)
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