When I try to change color of a JTextArea using
textArea.setText(\" Hi \")
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don't to use JTextArea for Html formatted Object (even is possible, but why bothering)
use JTextPane / JEditorPane instead (setContentType("text/html"))
notice, todays Java and methods implemented in Official API by Oracle supporting only reduced Html syntax with css <= Html 3.2
use JavaFX in the case that you want/need to use todays Html5,