Issue 16471 (#16489)

* fix: Label updateColor() and underline color work as expected

fixes Github issue #15214 correctly. underline uses _displayColor
and not _textColorF. _textColorF is only for the text.
This also emulates the SystemFont underline behavior

fixes Github issue #16471

* docs: added more internal doc for issue_16471

cocos/2d/CCLabel.cpp

@@ -1945,6 +1945,9 @@ void Label::updateDisplayedOpacity(GLubyte parentOpacity)
     }
 }
 
+// FIXME: it is not clear what is the difference between setTextColor() and setColor()
+// if setTextColor() only changes the text and nothing but the text (no glow, no outline, not underline)
+// that's fine but it should be documented
 void Label::setTextColor(const Color4B &color)
 {
     CCASSERT(_currentLabelType == LabelType::TTF || _currentLabelType == LabelType::STRING_TEXTURE, "Only supported system font and ttf!");

tests/cpp-tests/Classes/LabelTest/LabelTestNew.cpp

@@ -3197,7 +3197,7 @@ std::string LabelIssue15214::title() const
 
 std::string LabelIssue15214::subtitle() const
 {
-    return "Font and underline should be of the same color";
+    return "Font + underline: same color with setColor()";
 }
 
 //