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.

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.

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 of about equal length. Sometimes white is significantly more than 1/3rd, 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.

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.