Annie,
I've not seen any rigorous discussion about the exact mechanism behind tone differences in platinum palladium printing. But it is a fact that a warm temperature potassium oxalate developed pure palladium print will be warmer toned than a cold temperature ammonium citrate developed platinum print.

As far as the color differences go, I wonder if it has to do with the size of the platinum and palladium particles that are created during the redox reaction? The tones of silver gelatin prints have to do mostly with the size of the silver particles. Small silver particles have different optical properties than large ones. Warm toned developers (and lith printing) in silver gelatin processes create the warm tones by allowing the particles to remain very small. As the particles get larger and accrete to one another, they become more neutral black. Perhaps the palladium metal particles remain smaller and thus look warmer toned than the larger particle platinums. I don't know, and this is just a sheer speculation. SWAG (scientific-wild-ass-guess).

That said, I think some worker's 'warm toned' prints are warm because of inadequate clearing of the ferric oxalate. This is particularly prone to happen with printers who do not use border masks and allow the brush stroked areas outside of the image area to receive maximum exposure and go black. Adequate clearing is the big un-discussed achilles heel of the process, and the more work I see, the less convinced I am that it is being done consistently.