Since I don't quite understand what you want, I'll explain how the color change works. The maximum fuel stack size and the amount of fuel in the tank are taken, and from these two numbers, a percentage is calculated. This percentage is then used to set the position for the progress bar. You have the option to set your own limits in the config file under the 'List of Gauge Indicators' section.
Here is an example of a list of colors for different ranges. You can specify more or fewer values, but it's important that they do not overlap and that the first value equals the last value of the previous range.
"List of Gauge Indicators": [
{
"MaxRange": 1.0,
"MinRange": 0.9,
"Color": "#B1C06E"
},
{
"MaxRange": 0.9,
"MinRange": 0.8,
"Color": "#F7BB00"
},
{
"MaxRange": 0.8,
"MinRange": 0.7,
"Color": "#F70000"
},
{
"MaxRange": 0.7,
"MinRange": 0.6,
"Color": "#008080"
},
{
"MaxRange": 0.6,
"MinRange": 0.5,
"Color": "#4B0082"
},
{
"MaxRange": 0.5,
"MinRange": 0.4,
"Color": "#00FFFF"
},
{
"MaxRange": 0.4,
"MinRange": 0.3,
"Color": "#800080"
},
{
"MaxRange": 0.3,
"MinRange": 0.2,
"Color": "#FF00FF"
},
{
"MaxRange": 0.2,
"MinRange": 0.1,
"Color": "#00FF00"
},
{
"MaxRange": 0.1,
"MinRange": 0.0,
"Color": "#FFA500"
}
]