I don't know about alternate columns necessarily, but here's what I used to shade specific columns. For readability, every third or even fourth column is actually quite sufficient.
First, build a colour format with something like
Proc format:
value $bkgrnd
"Lvl1','Lvl2','Lvl3'='cxf6f5ea'
other='cxffffff';
run;
Then in proc tabulate, under the ODS umbrella, apply the $bkgrnd format to the class variable that contains the values of Lvl1, Lvl2,Lvl3,... with
classlev LvlVarname/style=[background=$bkgrnd.];
Not my invention, I think Cynthia helped me through this one. You should be able to find more in the archives in the ODS section.
Hope this helps at least a little bit!