Unfortunately it seems, for now, that you will have to rely on trial and error alone - you may get lucky! Otherwise, you could forget about the wild watermelon shirt for now and head to www.squidoo.com/KinzStyle, which has a great selection of combinations to make a number of other garments.
When searching for the recipe for the wild watermelon shirt, you must be careful of the sites you may visit. Many will advertise that they have the information; however a lot of sites are simply loaded with a variety of key words in order to attract innocent web-surfers. When you access these sites, there is generally no relevant content at all. These sites can potentially carry viruses and so should be avoided at all times.
Try to use trusted sites and Webkinz forums to get your information. You should find lots of great tips and discussion on some of these pages. The forums can be found if you head to Google and type in 'Webkinz forums'.