I have a native NTSC 1084S and a native PAL A600. Over composite, the A600 display is black and white, but over RGB it's full color. Could be a nuance of this particular era of 1084Ses - mine's one of the later ones, a 1084S-D2, I think.
I can't remember what happens if I connect a native NTSC Amiga over composite and set the screenmode to PAL...
I used to have an older NTSC 1084 with an NTSC CD32 connected over composite. I think I was able to set that one to PAL in software and it still worked, but I can't really remember.