Enamel paints will hold up better outdoors. Look for them at Michael's, Joann's or Walmart in the decorative paint section. Or, by the terra cotta pots at Michael's, you'll find "Patio Paints" which were especially made for using on the pots which are going to be watered.

Is this patio paint permanent? I'm thinking what if our address# should change. It did once when they put the 911 system in our area. Do you think painting them directly onto my vinyl siding would look ok, or tacky? Would you do it yourself, that is if you have vinyl siding?

I guess rather you paint on your sideing or not would be a personal choice, since most sideing is not stright, from a side view it might not look how you may be thinking, why not use a peice of wood to put your house # on or even a large stone, just a couple of ideas.
oddie

I do have an unfinished wooden plaque that I bought at Walmart a little while ago. I'm going to either use that or stencil the numbers onto my mailbox.

oddie, like you said, some siding is not straight from the side view, and mine isn't. I really don't want nail holes in my siding, but, how would those straight numbers bought at a hardware store look on not so straight siding??