Do you think its better to buy turquouise icing for a cake or tint white icing myself for it? Which gives best results??
Also how do you stop icing cracking once on a cake?

to stop the icing from cracking you need to use a good stable base to minimize the cake from moving. As far as the coloring... I'd just color it myself assuming that you are talking about buttercream. If your talking fondant it depends on if your time constraints