If I'm not mistaken, you should be able to accomplish this. I'm not in front my gaming PC to test though, just fyi.
Find the Grid setting that changes the unitframes' color, add the dispellable debuff indicator to the unit frame background indicator setting and make sure it's at the top of the list of indicators. Now, the unitframe backgrounds should change when there is a dispellable debuff.
If you can't get it to work, reply here and I'll get you a screenshot to help (once I'm home).
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