There's a wonderful tutorial on drawing 3D graphics in PostScript on 
http://www.math.ubc.ca/~cass/graphics/manual/, specifically
http://www.math.ubc.ca/~cass/graphics/manual/code/doughnut.eps as an 
example.

And there's a detailed description on shading in PostScript in
http://partners.adobe.com/public/developer/en/ps/PLRM.pdf, page 259
(I attached an example and have some more on computer if anyone is 
interested; I downloaded them from Adobe, but don't know the URL any 
more) or on shading in PDF in
http://partners.adobe.com/public/developer/en/pdf/PDFReference16.pdf, 
page 272. (They're basically the same.)

In metapost, "perfect" spheres can be drawn, like on 
http://www.gnu.org/software/3dldf/cdsmpls.html. But how can I add colour?

Even if it would be too tricky to add colour and proper shading on the 
sphere - Is there any possibility to use advanced shading techiques 
described in PLRM inside metapost/metaplay for the most simple cases, 
say, like in the attached example? Metaplay allows circular and gradient 
shading and no others. I don't mind if I had to hard-code the shading.

PSTricks allows some more advanced shading techniques and even direct 
PostScript code integration (with \special I think). Is anything similar 
possible in metapost/play?

The spheres drawn directly in PostScript as in the examples above still 
have straight lines (well, I agree that with some mathematics it would 
be possible to draw them with proper splines as well).

Thanks for all the suggestions,
	Mojca