Then you're using the legacy coloring, and I'm afraid that means you're going to be dealing with past difficulties I'm glad to have seen the back of. IIRC, if you make the background colours (panel and item) the same and choose the right tint strength settings then the compositing should match, but I have recollections that even then it would look wrong in some circumstances.
That's correct, you'd need to set that colouring field manually using image transparency, which is more difficult but will provide predictable results.