What do you mean by the "keyboard changes" - question is so unclear I cannot imagine what you are referring to... something in the screen perhaps (or are you under disco club lighting?) 🙂

You realize that the color scheme of your Motif XF operating system is customizable
Press [UTILITY]
Pres [F1] GENERAL
Press [SF2] SCREEN
COLOR can be set as you prefer
Press [STORE] to write this preference to your instrument's Flash ROM
This will be written into each ALL data FILE you create from this point forward and is stored as apart of the ALL file's SYSTEM settings.

What this has to do with "splitting Voices" is not known - what do you mean by "splitting Voices" are you referring to making a SPLIT in PERFORMANCE mode?

In the performance mode, go to job and voices. The four voices are on the screen. When you move the cursor to the key numbers e.g. C-2 and G8, as you advance through the key numbers the keyboard changes colors. White to dark red to grey. I think is has to do with where the keyboard is splitting.

Oh, much better description!!! 🙂

Yes the MIDI keyboard consists of 128 notes... from C-2 through to G8.
A regulation piano (88 key) Motif XF8 has notes from A-1 through C7
A Motif XF7 has notes from E0 through G6
A Motif XF6 has notes from C1 through C6

So what you are seeing is the overall MIDI keyboard from C-2 through G8... then you are seeing a color change to indicate your particular Motif XF 6, 7 or 8, then if a PART is set to a specific region, that is indicated by the color - white area does not sound.

Middle "C" = C3 on Yamaha keyboards.
When a keyboard graphic appears in the screen you typically set the area that this PART controls... be it for making sound (VOICE) or whether it is the area that will trigger the ARPEGGIATORs (ARP)

Hope that helps.