Wow so glad there is so much interest in things Jewish (you would not think so from SU's selection of stamps). I work at a conservative synagogue so if anyone has a real technical question I can get them an answer! I have used Kosher Creative and they have lost of fun charms you can put on cards! As for Bar or Bat Mitzvah card colors, I try to use the same colors that they used on the invitation, I have even taken cards apart and used parts of them on the card (if I ever get around to posting cards I will inclued the Bat Mitzvah cards I recently made). I am trying very hard to get out of the use of blue and silver - I just checked with my sister (Sunday school teacher) and the only reason blue and silver are used is to mimic the colors of a Tallit (prayer shawl) which must contain a blue thread. This is the same reason the Israeli flag is blue and white.
As for Bar & Bat Mitzvah gifts I tend to stick to US savings bonds!