Do you have any piping nozzles? I am wondering how much of a novice you are 
It looks to be a basic buttercream coating, then buttercream coloured with a few different colours piped on with about 3 different nozzles, the green looks to be a leaf, one is just an open round nozzle etc. There are loads of videos on YouTube showing how to pipe basic shapes. As it is buttercream you can always just wipe it back off and put it back into the bag!
The colours on top that look like water colour are just smears of the coloured buttercream.
You can practise on a plate before piping on a cake. Go for it!