That's easy. Here you have a code snippet that adds the Times Roman text in Blue and the Chinese text in Red:
FontSelector selector = new FontSelector();
Font f1 = FontFactory.getFont(FontFactory.TIMES_ROMAN, 12);
f1.setColor(BaseColor.BLUE);
Font f2 = FontFactory.getFont("MSung-Light",
"UniCNS-UCS2-H", BaseFont.NOT_EMBEDDED);
f2.setColor(BaseColor.RED);
selector.addFont(f1);
selector.addFont(f2);
Phrase ph = selector.process(TEXT);
In your case you need two FontSelectors.
FontSelector selector1 = new FontSelector();
Font f1 = FontFactory.getFont(FontFactory.TIMES_ROMAN, 12);
f1.setColor(BaseColor.BLUE);
selector1.addFont(f1);
Phrase ph = selector1.process(str1);//First one
FontSelector selector2 = new FontSelector();
Font f2 = FontFactory.getFont(FontFactory.TIMES_ROMAN, 12);
f2.setColor(BaseColor.GRAY);
selector2.addFont(f2);
Phrase ph = selector2.process(str2);//Second one
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