In testing my project I discovered that Reaper rendered the AU differently from the 2.10.0 IDE and Logic Pro, as you can see in the screen grab comparison.
Turns out Reaper has problems with alpha channel transparency for groupboxes and the form, which I was using to get the shades I wanted. Fix was to strip the color from the Form and add a PNG background using the color formerly assigned to the Form.
Hope this helps anyone who also runs into this.