I think it's instructive to look at passenger vehicle AC/heater controls. These often come in the form of a circular dial whose opposite ranges are denoted red and blue. How they treat the middle between them falls into several patterns:
no "lukewarm" zone: abrupt transition from blue to red, with perhaps a small gap.
no "lukewarm" color, but a somewhat larger gap.
"faux cross-fade" from red to blue with overlap: one stripe is narrowing as the other widens.
"faux cross-fade" with coarse-grained dithering: the stripes are chopped up and blended, with cuts of one color getting longer as the others get shorter.
The stripes meet, or nearly meet, without overlap, but get thinner toward the center meeting point.
The "lukewarm" zone is indicated as a white stripe. Sometimes the blue/white/red arcs are of about equal length. Sometimes white is significantly more than 1/3rd of the range, sometimes significantly less.
I'm unable to find a picture of a design that uses a third color other than white or else transparent (whatever the background/substrate color is).
In a computer UI I'd probably make a gradient from red to white to blue, or from red to transparent/background to blue, or else imitate the above ideas.