How about using one of several colour pickers made for the web? You can use a WebViewer to show them and use the WebViewString to send the colour data to the blocks workspace
that example was working fine…
I also was using it succesfully here App Inventor Extensions: Tools | Pura Vida Apps to test the StatusBarColor, NavigationBarColor and TitleColor blocks of the tools extension
so if it is not working for you, you might want to add a screenshot of your relevant blocks, so we can see what you are trying to do, and where the problem may be.