Hello Nidhi, 1. Purple fringing is usually called Chromatic Aberration. Usually caused by inferior lenses. You can see this phenomenon when taking photos of subjects with great contrast. It looks like a fine line of discoloration (usually purple in color) on the edge of your subject.
2. Lithium batteries are better than AA batteries. They hold power longer and can put out a constant voltage throuout its lifecyle until the charge runs out. It is lighter and holds power a lot longer. Lithium-Ion batteries are different from regular Lithium Batteries.
3. Who told you that you need a dummy battery? You only need a dummy battery if you are using the optional AC adaptor. Its purpose is to make contact with the terminals inside the battery compartment and the AC adaptor's power terminals. Other than that, the charger is included with your purchase and a dummy battery is not necessary.