Tie dyed fondant is a bit tricky. No two pieces will ever come out the same. The fondant is more marbleized than Tie dyed. I suggest getting a few colors of fondant you would like to use and placing them side by side. Roll the fondant in a ball, cut that ball in half once or twice and place it side by side again. Now roll the fondant in a ball again, than roll the fondant flat to the thickness that you need.
Airbrushing cakes is one of the easiest ways to decorate. To successfully get the look of Tie dye on buttercream,I suggest first putting your cake on a turntable and having your icing as smooth as possible. Now choose the colors you plan on airbrushing with and start off with the lightest color first. I always start with yellow. I make a thin w pattern in a circular (swirl) motion around my cake with the yellow, once I complete this I start with my next color leaving a tiny bit of white in between the yellow and the color I am using. I continue this until I use all of the colors I planned to and my cake is completely covered in Tye dye. It is okay if your colors blended together not all Tye dye is the same. You can also use white airbrush to lighten up areas where you would like it to be a little lighter. My cakes are usually bright tie dyes as I recommend against pastels because the effect does not come out the same.
It is such a huge surprise finding out if your having a boy or girl. Did you ever think why not share that surprise with all your friends and family at the same time? You now can! Have your doctor write on a piece of paper what your having and take it to a local bakery. There they can either dye the cake to the color of child your having or fill it with colored icing! This way when you cut into the cake everyone will find out at the same time whether your having a boy or a girl or in some cases both! We call this the Baby surprise cake!