fix https://github.com/NanoVNA-Saver/nanovna-saver/issues/596

Signed-off-by: Martin <Ho-Ro@users.noreply.github.com>

src/NanoVNASaver/Windows/DisplaySettings.py

@@ -551,6 +551,7 @@ class DisplaySettingsWindow(QtWidgets.QWidget):
             logger.info("Invalid color")
             return
 
+        setattr( Chart.color, attr, color ) # update trace color immediately
         palette = sender.palette()
         palette.setColor(QPalette.ColorRole.ButtonText, color)
         sender.setPalette(palette)