Unfortunately your tip does not work for me. I will illustrate the steps to highlight the issue that I have.
The following is the Start11 image I am using. It is a png image, the white area being transparent.

I set the image in Start11 per your instructions with the current accent colour being a blue...

The Start11 menu colours as expected...

I normally have my wallpaper set to change periodically. Here I force change the next background...

The new wallpaper generates a red accent that my Curtains skin automatically changes to...

The Start menu image remains blue...

I now select the settings menu in Start11, without even selecting an option...

And my start menu image recolours to red, matching my Curtains skin...

This is the outcome I want, but I cannot seem to get it to happen automatically when my wallpaper changes and causes a new accent colour to be selected. My Curtains skin recolours as soon as the wallpaper changes, but not the image on my start menu. It seems to me that it's a bug.