Acrylic craft paint is excellent for that. All craft stores have it. So does Wal-mart and some dollar stores, though take my advice and but Delta Ceramcoat or Apple Barrel. The others I've tried(Craftsmart comes to mind) are so thin you need 4 coats to cover. It's what you probably should have bought in the first place, since it comes in small bottles and many colours, and is suitable for exterior use. Unless you happen to have some acrylic paint around the house, there is nothing around the house that you can use to colour paint with. Food colouring will do the same as the poster paint, so don't try that.

I want to paint a design on a wood bird house, and would like to have a few colors to do so. But I don't want to have to pay for a whole can of each color when I only need a tablespoons worth.

I got a pint of White acrylic outdoor, house paint. (covered the bird house)

When I tried adding "poster paint" the color washes out after the rain leaving the house white.

Is there anything I might have around the house I could use to tint it?