When I am decorating a cake that I plan on airbrushing, I practice layering different colors on a white piece of paper to see which combo looks like the color I need. I see to always need to layer when i am doing an all over color.
I would stay away from any baked/decorated goods. Stick to buying something she needs off the registry and not make this a competition. As jmt1714 said, the shower is for the mom to be and you are their to celebrate her and her new baby.
I like the I DO cake...the car on it is PERFECT for someone getting their liscence! I would do something very similar, different colors like aqua, white and little pale yellow. Not too frilly with those colors, but still girly colors.