1. Chocolate.
I piped a picture of a dog on my son's 2nd birthday cake with just chocolate frosting. Vary the shades by adding more or less chocolate.
2. Red dragon fruit
Cut a small piece and squeeze the juice out using a sieve. It gives out a light pink shade to your frosting.
3. Turmeric powder
For a bright yellow shade. Be careful not to use too much or your frosting will taste like dhal.
4. Purple cabbage.
You'll need a good juicer for this, for a light lilac shade.
5. Avocado.
For a very nice pastel green shade. Best used immediately. The colour would change to a darker shade of green if you leave it overnight, because of oxidation.
Other foods you can try are, carrots, beet root and pandan (screwpine) leaves. Don't expect vibrant colours just natural goodness!