[Xy-pic] color output with xy?

[Roger Hart]

I have what I'm afraid is another novice question about xy, and  
xymatrix in particular. For a presentation I am working on, I would  
like to set xy so that it produces everything in white, against my  
black presentation background. I'm using Mac OS X, TeXShop, ps4pdf,  
and xy. I'm using \usepackage[dvips,ps,all,color]{xy}. I've tried  
various things but without any success. Is there a way to use add one  
command to globally change the color to white in all my xy pics? I  
don't want to change each object, one at a time. Thanks for your  
patience,

Roger


PS. The larger problem, posted on OS X TeX

I would like to use Keynote's "Gradient" background (which is dark  
grey->black) for a presentation I'm preparing, which means, if I'm to  
cut-and-paste, that I need to have my text and equations in white. I  
use the latest version of TeXShop and KeyNote.

Ideally, I would hope that there is some global command I could use  
that would reverse the usual colors once and for all in the entire  
document, typesetting my entire article in white text, and the entire  
background in black, and overriding any color assignments I've made  
in the article. This may be complicated because I use ps4pdf and xy  
quite a bit.

I've looked into the seminar package, and some other approaches,  
without much success. I'd like to find something that would just  
allow me to very quickly produce Keynote presentations from my  
articles,  without a lot of extra work. Any suggestions?


********************************

Roger Hart
Assistant Professor, Departments of History and Asian Studies
University of Texas at Austin