Ok so I made an adult cake for a friend of mine and used buttercream frosting on it. This party was held outside and after about an hour the frosting started melting and looking very watery.
What is the best kind of frosting to use when using the cake decorating tips. And does that also work when I apply the food coloring? Please help. 
Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:
http://www.cakecentral.com/cake-decorating-ftopict-605188-.html
Above super thread has popular CC recipes for crusting American buttercreams (Crisco and hi-ratio recipes), several types of fondant and doctored cake mix (WASC and other flavor variations) - and so much more!
American b/c's made with all butter will not hold up as well as 1/2 butter & 1/2 shortening, which will not hold up as well as 100% shortening recipe. But American b/c's made with hi-ratio shortening will hold up the longest.
You also don't want to use the liquid food coloring found in grocery stores to color your frosting.
Concentrated paste/gel colorings formulated for cake decorating work best:
http://www.globalsugarart.com/cat.php?cid=858&s=&name=AMERICOLOR/Americolor%20Gel%20Paste%20Colors
http://www.wilton.com/store/site/department.cfm?id=3E305008-475A-BAC0-50998F2253BBBC1F&fid=7816D930-475A-BAC0-5CB6ADF25218B917
HTH
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