Changing Transparency of Visual

Hello,

I have 4 visuals on my page reporting data. One of the things I'd like to do is make one look "dim" when there isn't data present so the user's eye is attracted elsewhere. The visuals are non-interactive, so I thought I'd toss a rectangle shape on top of the visuals.

However, it doesn't appear that I can control the transparency of the background. I looked here and here, both of which talked to using an 8 character hex (so #b3b3b350) code in a measure. 

I tried that without success, if the measure was ##ff0000 - I get what I expect, bright red with 0% transparency. When I try #FF000050, it's treated as though it's 100% transparency. This functionality appears to work on the data label color field in a card. Thus, I didn't know if one couldn't apply that transparency to backgrounds.

 With all of the improvements with using measures across the visuals, I wonder if it's one of those things I need to put something into the "idea box" to allow for background transparency changes as well as extending the measures to fill and line colors in shapes.

Thanks in advance