I am doing a birthday cake for a little girl and her favorite color is purple. She has asked if I could make the "inside" of her cake purple (meaning the cake batter). My thought was to do a marble effect with white cake and mixin purple batter. Can I just add food coloring to white cake mix?? will adding a small amount of coloring effect how the cake bakes? Has anyone tried this before that can offer tips? The cake is for this weekend. Thanks for the advice!

Yes you can add food colouring to the batter. Here's a tutorial on how to make a rainbow cake that will probably help you out:

http://www.omnomicon.com/rainbowcake

On the shark cake I did, the inside of the shark was red and the inside of the water was blue, just used regular gel coloring added to the cake batter, tasted fine

Coloring your cake batter is completely fine...mix in a few drops at a time or small quantities at a time till you get the color you want. Mixing in color to your batter will not affect the consistency...so no need to worry. Also, from my experience, and its just a suggestion... if you want the child to be 100% happy, give her what she asked for... i.e; a purple fully purple cake...don't marbel the batter...coz she's gonna be expecting a fully purple interior when she cuts it.