Use a standard white cake mix and prepare according to the box for this Barney birthday cake. Add Violet concentrated paste (in the cake decorating part aisle of Wal-Mart) to the batter until it looks as purple as you want it to be. Bake the cake and allow sufficient time to completely cool off.
While it's cooling, prepare your icing with the different colors needed. I used the basic food dye to make a batch of green icing. I used the violet paste to make a light purple and a darker purple. I saved some white icing too.
When the cake is completely cool, spread on the green icing nice and thick so no crumbs get mixed in (use a little bowl of water to dip the spatula in occasionally).
For Barney I cut out a picture of him (with a fun surprised look) from a little coloring book I got at the Dollar Store. Gently place the Barney cutout on the cake where you want him. Use a toothpick to make a line in the icing around Barney. Remove the cutout and use a butterknife to remove the icing from the cake where Barney will be. Put the cutout on the cake again and use the toothpick to poke holes through the cutout and into the cake where lines meet to give yourself a good layout to work with.
Use the holes in the cake and the actual cutout as a guide to fill in the colors. Outline the shape of Barney and his inside lines with the darker purple. Use light purple to color his body. Use the green to color in his belly, his spot on the tail, and the spots on his toes. Use white to color a line of teeth and his eyes. Use the darker purple to put pupils on his eyes. I also used the different colors to put my child's name and her age, swirlies around the sides, and random dots all around the cake.
Everyone thought it was pretty awesome for a homemade cake and it really wasn't that difficult. And seeing a vibrant purple cake underneath the green icing was pretty surprising to everyone too!