Just picked up a MasterKeys Pro S RGB.
When in "Single Key" LED mode, with "Key" set to green, and "Rest" set to blue, tapping the key does the expected. It makes the blue key green. However, once the key fades out, it's a stark, immediate illumination of blue after the fade out. It's very jarring and sort of kills the fading experience you've got going on with this mode. Of course, when "Rest" is set to no color, things look fine.
So, one of two suggestions:
Allow or change the fade so that instead of fading out, then immediately turning back on to the original color, fade between the colors. Or..
When "Rest" is selected as a color (non-black), fade the illuminated key out, then fade it back in to the original color vs the immediate turn on.