Can't set color to transparent in business rules

So I'm trying to set the color of an object based on certain criteria, and to clear it if those criteria are no longer met, but whenever I try to set the color to "transparent," it says there is an error and it can't save. Is there a reason for this? I can manually set it to transparent, just not in the business rules. I am sure it's the fact I'm choosing transparent that is causing the issue, because if I change the color, it works just fine.

Here is a screen shot of the color and error message: