Diffuse colours on materials typically come from within the material, while the specular colour is from the very surface. Coloured plastic materials are made by embedding particles of dye inside a colourless medium, so the diffuse colour is the colour of the dye, while the specular colour is white from the colourless surface. With metals, all of the reflection is at the surface, so the diffuse and specular colours are the same.
There are many different kinds of paint, but most commonly they're made like plastics: putting dye particles inside a white or colourless medium. Emulsion, acrylic, and enamel paints are all made this way. For this reason, painted or varnished metal surfaces have white specular as if they were plastic. There are some kinds of paint (metallic paints being the most obvious example) that aren't made this way, so it's not always true.
Ambient doesn't correspond to a physical property of the material: it's a cheap substitute for global illumination. It represents all of the indirect light that falls on the shading point and is reflected in the viewing direction. There's no physically correct way to set it: you should just do what looks best in your scene. That said, I'd offer a rule of thumb that shinier objects probably want less ambient, because less of the indirect light is going to be reflected diffusely to where you can see it. However, you might consider not using ambient at all, and pointing dim fill lights at places that will come out too dark without indirect light.
