Hi Daphne,
I have a tutorial how to change the collor of blank cells here
http://ewbooks.info/hotpot/jcross-change-cell-background
You have two options
1: edit the output web page exercise .htm and upload it as a HotPot activity into Moodle.
Or
2: modify the source hp6.cs_ in your source folder of your installation of HotPotatoes on your computer. Then upload the .jcw source file AND and the hp6.cs_ as HotPot activity in Moodle.
Both approaches work.
Cheers
Stan