If I had to guess, it’s mostly water.

The really scary stuff (in aqueous solutions, anyways) isn’t usually brightly colored, it’s usually clear. Of course there are exceptions - like bromine or iodine - but I know what those look like and nothing on that tree looks like either of those.

The reason for that is that most pigments are conjugated ring systems - complex organic molecules that break down in harsh environments - so anything sufficiently reactive would actually destroy any colorants you added to them.

I’m not saying that bright, friendly colors are always safe… but the easiest way to get a bunch of bright colors is to use food coloring and water.