I'm assuming you are taking something like a toilet waste down the side of a house to below ground drains. As far as I know you can do this as long as the pipe is securely and adequately supported (eg so the joints wont pull out). You don't have to paint it unless you want to. Just thought, there may be an issue with UV degradation. I seem to recall that pipework exposed to sunlight has to be a different 'grade'. Check with your local builders merchant.
You are correct sddsddean whitejack, The orange pipe work does not have the same UV protection as the other types of soil pipe and will turn brittle and weak if exposed to the suns UV rays. As for painting it to protect it, I am not sure. As there is very little difference in price is it worth the risk ???