@Tomahawk right, hopefully this will all be clear enough 
To change the badge colour, we use theme, and define the colours as hex:
homeassistant:
customize:
sensor.bedroom_temperature:
templates:
theme: >
if (state > 25) {
return 'hot';
} else if (state >= 18 ) {
return 'comfortable';
} else {
return 'cold';
}
frontend:
themes:
cold:
primary-text-color: '#447ebc'
label-badge-red: '#447ebc'
comfortable:
primary-text-color: '#629e51'
label-badge-red: '#629e51'
hot:
primary-text-color: '#bf1b00'
label-badge-red: '#bf1b00'
That will produce this: 
If we then add the sensor to a group, with the above left in place, it will change the text and state to the relevant colour, as shown here:
If we want to just change the icon colour, then we use rgb_color and define the colours as R, G and B values on a 0-255 scale:
homeassistant:
customize:
sensor.bedroom_temperature:
templates:
rgb_color: >
if (state > 25) {
return [191,27,0];
} else if (state >= 18 ) {
return [98,158,81];
} else {
return [68,126,188];
}
You can easily convert between RGB and Hex using this site:
Hope that helps!