How can I keep selected text hilited?

[Robert E. Ball]

I want the user of my program to be able to hilite portions of a text  
field on a given card (similar to using a yellow hilite pen on a book  
page -- I have set hiliteColor to yellow) and then to have the  
hilited text show up when the user returns to the card.

When I first double click on a word in a field, the selected word's  
backgroundColor turns yellow, as it should. However, when I double  
click on another word in the field, the yellow backgroundColor of the  
previously selected word is removed and the backgroundColor of the  
second selected word is yellow. I understand why this works -- a new  
word has been selected. However, is it possible to write a script to  
save the yellow backgroundColor of all selected words. In other  
words, I would like my cards to look just like the page of a book  
that has hilited text.