Well, anything really!!
I always point people in the direction of my cake smash blog page for inspiration. There are lots of ideas and colour themes to get you started.
I am always here to run your ideas past and I have lots of experience to let you know what works and what doesn't.
I have lots of buntings and decorations and If there is something I don't have.... well, I usually just make it!

I use Crayola paint.
I generally don't like using natural paint such as food colouring and yoghurts as I like the little ones having their canvases to keep. I think the natural paint will 'go off' and can't be kept .