Generate colors between red and green for a power meter?

Question

This should work - just linearly scale the red and green values. Assuming your max red/green/blue value is 255, and n is in range 0 .. 100

R = (255 * n) / 100
G = (255 * (100 - n)) / 100 
B = 0

(Amended for integer maths, tip of the hat to Ferrucio)

Another way to do would be to use a HSV colour model, and cycle the hue from 0 degrees (red) to 120 degrees (green) with whatever saturation and value suited you. This should give a more pleasing gradient.

Here's a demonstration of each technique - top gradient uses RGB, bottom uses HSV:

Off the top of my head, here is the green-red hue transition in HSV space, translated to RGB:

blue = 0.0
if 0<=power<0.5:        #first, green stays at 100%, red raises to 100%
    green = 1.0
    red = 2 * power
if 0.5<=power<=1:       #then red stays at 100%, green decays
    red = 1.0
    green = 1.0 - 2 * (power-0.5)

The red, green, blue values in the above example are percentages, you'd probably want to multiply them by 255 to get the most used 0-255 range.

Short Copy'n'Paste answer...

On Java Std:

int getTrafficlightColor(double value){
    return java.awt.Color.HSBtoRGB((float)value/3f, 1f, 1f);
}

On Android:

int getTrafficlightColor(double value){
    return android.graphics.Color.HSVToColor(new float[]{(float)value*120f,1f,1f});
}

note: value is a number between 0 and 1 indicating the red-to-green condition.

If you want an green-yellow-red representation like the accepted answer suggests then take a look at this.

http://jsfiddle.net/0awncw5u/2/

function percentToRGB(percent) {
    if (percent === 100) {
        percent = 99
    }
    var r, g, b;

    if (percent < 50) {
        // green to yellow
        r = Math.floor(255 * (percent / 50));
        g = 255;

    } else {
        // yellow to red
        r = 255;
        g = Math.floor(255 * ((50 - percent % 50) / 50));
    }
    b = 0;

    return "rgb(" + r + "," + g + "," + b + ")";
}


function render(i) {
    var item = "<li style='background-color:" + percentToRGB(i) + "'>" + i + "</li>";
    $("ul").append(item);
}

function repeat(fn, times) {
    for (var i = 0; i < times; i++) fn(i);
}


repeat(render, 100);

li {
    font-size:8px;
    height:10px;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
<ul></ul>

Linearly interpolating between green and red almost should work, except that in the middle there will be muddy brown color.

The most flexible and best looking solution is to have an image file somewhere that has the exact color ramp you want. And then lookup the pixel value in there. This has the flexibility that you can tweak the gradient to be just right.

If you still want to do it from code, then it's probably best to interpolate between green and yellow colors in the left side, and between yellow and red on the right side. In RGB space, green is (R=0, G=255, B=0), yellow is (R=255, G=255, B=0), red is (R=255, G=0, B=0) - this is assuming each color component goes from 0 to 255.

I made a small function wich gives you the rgb integer value for a percentage value:

private int getGreenToRedGradientByValue(int currentValue, int maxValue)
{
    int r = ( (255 * currentValue) / maxValue );
    int g = ( 255 * (maxValue-currentValue) ) / maxValue;
    int b = 0;
    return ((r&0x0ff)<<16)|((g&0x0ff)<<8)|(b&0x0ff);
}

So if your currentValue is 50 and your maxValue is 100, this function will return the color that you need, so if you loop this function with a percentage value, your color value will go from green to red. Sorry for the bad explanation

I'd say you want something in between a line segment in the HSV space (which has a slightly-too-bright yellow in the centre) and a line segment in the RGB space (which has an ugly brown in the centre). I would use this, where power = 0 will give green, power = 50 will give a slightly dull yellow, and power = 100 will give red.

blue = 0;
green = 255 * sqrt( cos ( power * PI / 200 ));
red = 255 * sqrt( sin ( power * PI / 200 ));