how to use patch and diff to merge two files

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http://unix.stackexchange.com – I have read about diff and patch but I can't figure out how to apply what I need. I guess its pretty simple, so to show my problem take these two files: a.xml <resources> <color name="same_in_b">#AAABBB</color> <color name="not_in_b">#AAAAAA</color> <color name="in_b_but_different_val">#AAAAAA</color> <color name="not_in_b_too">#AAAAAA</color> </resources> b.xml <resources> <color name="same_in_b">#AAABBB</color> <color name="in_b_but_different_val">#BBBBBB</color> <color name (HowTos)